Method and apparatus for reducing a nitrogen oxide

ABSTRACT

Disclosed herein is a method and apparatus for reducing a nitrogen oxide.

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/389,866, filed on Jun. 19, 2002, which isincorporated in its entirety as a part hereof for all purposes.

FIELD OF THE INVENTION

[0002] This invention relates to methods and apparatus for reducing anitrogen oxide. In particular, it relates to the use of a gas analyzerto obtain information related to the compositional content of amulti-component gas mixture that contains a nitrogen oxide for thepurpose of assisting in the control of the reduction.

TECHNICAL BACKGROUND

[0003] Oxides of nitrogen (NOx) that are emitted by an emissions source,such as those formed as a result of combustion, are included among themain causes of the “acid rain” problem, the photochemical smog problemand the resulting damage to the environment. These harmful substancesshould therefore be eliminated to the greatest extent possible from thegases emitted by an emissions source, such as the exhaust from acombustion process, prior to their discharge into the atmosphere.

[0004] One source of nitrogen oxides, in the form of NO₂ and mainly NO,are those formed by the combustion of coal, oil, gas, gasoline, dieselfuel or other fossil fuels. Combustion of fossil fuels occurs, forexample, in a stationary device such as furnace, which is a device forthe production or application of heat. A furnace may be used inconnection with a boiler such as in a steam generator that drives asteam turbine in an electrical generating plant, in connection with anindustrial operation such as in a smelter or chemical reactor, or inconnection with supplying heat for human consumption.

[0005] Fossil fuels are also combusted in a mobile device, including adevice that supplies mechanical power such as an internal combustionengine in a vehicle for transportation or recreation, or in a piece ofequipment for construction, maintenance or industrial operations; or ina gas turbine, which is a turbine driven by a compressed, combustedfluid (such as air), such as in the engine of a jet aircraft.Gas-emitting devices such as an internal combustion engine or a gasturbine are also found in stationary applications, however. The exhaustgas emitted by devices such as those described above is amulti-component mixture of gases containing nitrogen oxides. Nitrogenoxides are also emitted by plants for the incineration of industrial ormunicipal waste. In addition, carbon monoxide and hydrocarbons are alsoemitted by these sources.

[0006] A problem exists with respect to the need for control of theinjection of a reducing agent into a gas mixture containing nitrogenoxides. There is a desire to effect the reduction of as large a quantityof the nitrogen oxides present in the gas mixture as possible. For thispurpose, what amounts to a stoichiometric excess of reducing agent, interms of the quantity of nitrogen oxides present, is often injected intothe gas mixture and thus into the nitrogen oxides. An excess of reducingagent is employed not so much by design but primarily because of theunavailability of information related to the compositional content ofthe gas mixture sufficient to accurately calculate the stoichiometricequivalent of reducing agent needed. The compositional content of a gasmixture containing nitrogen oxides often varies in an extremelyunpredictable manner as it moves through a conduit from its emissionsource to the point of its ultimate destination, such as a point ofdischarge into the atmosphere. As a result, because of the desire toobtain reduction of a large percentage of the nitrogen oxides, an amountof reducing agent is injected that later proves to be an excess. Whetherthis results from calculations based on inaccurate or incompleteinformation, a strategy of employing an excess to be certain that toolittle is not employed, or incomplete reaction of whatever the amount,the same undesired consequence is experienced—unreacted reducing agentis discharged to the atmosphere and becomes a pollutant itself. Whenammonia is the reducing agent, this is known as ammonia slip. In a gasmixture that is unscrubbed, or otherwise contains sulfur oxides,unreacted ammonia is also capable of reacting with the sulfur oxides toyield corrosive, sticky deposits of ammonium sulfate and/or ammoniumhydrogen sulfate that foul the mechanism of the conduit.

[0007] There is a need then for a method and apparatus for the reductionof a nitrogen oxide that provides control of the reaction of reduction,and in particular control of the injection of a reducing agent into thegas mixture containing the nitrogen oxide. In particular, there is aneed for a method and apparatus that enables the calculation of theamount of reducing agent to be injected in relation to information aboutthe compositional content of the gas mixture.

[0008] This invention addresses those needs by providing a method andapparatus in which analysis of the gas mixture is performed to furnishinformation related to the compositional content thereof. In certainembodiments, the analysis is furnished by a gas analyzer that may beplaced within a conduit through which the gas mixture is transported inpositions that create an opportunity to develop useful information aboutthe gas mixture, and especially information related to the nitrogenoxide content thereof. In certain other embodiments, a gas analyzer isemployed for this purpose that outputs a signal related to the contentwithin the gas mixture of an individual component gas therein and/or thecollective content of a sub-group of gases therein. In certain otherembodiments, the information is inputted into a decision making routineand/or a map, and may be used to calculate a desired amount of reducingagent to be injected into the gas mixture, and thus into the nitrogenoxides to be reduced. Other embodiments of the invention are as moreparticularly described below, or are as would be apparent to the artisanin view of the description below.

SUMMARY OF THE INVENTION

[0009] One embodiment of this invention is an apparatus for reducing anitrogen oxide contained in a multi-component gas mixture emitted by aemissions source that involves (a) an exhaust conduit for transportingthe gas mixture downstream from the emissions source, (b) an injectorfor injecting a reducing agent into the conduit, and (c) one or more gasanalyzers located in the conduit downstream of the injector.

[0010] Another embodiment of this invention is an apparatus for reducinga nitrogen oxide gas emitted by a emissions source that involves (a) anexhaust conduit for transporting the nitrogen oxide gas downstream fromthe emissions source, (b) an injector for injecting a reducing agentinto the conduit, (c) a first catalyst to catalyze the reduction of thenitrogen oxide, (d) a gas analyzer located downstream from the firstcatalyst, and (e) a second catalyst to catalyze the reduction of thenitrogen oxide located downstream from the gas analyzer.

[0011] Another embodiment of this invention, in a multi-component gasmixture that is emitted by a emissions source and contains a nitrogenoxide, wherein a nitrogen oxide is reduced by injecting a reducing agentinto the gas mixture and contacting the gas mixture with a catalyst, isa method of determining the amount of reducing agent to be injected, orof decreasing the amount or release of unreacted reducing agent, bydetermining information as to the compositional content of the gasmixture, and controlling the injection of the reducing agent in relationto the information as to the compositional content of the gas mixture.

[0012] Another embodiment of this invention, in a multi-component gasmixture that is emitted by a emissions source and contains a nitrogenoxide, wherein a nitrogen oxide is reduced by injecting a reducing agentinto the gas mixture and contacting the gas mixture with a catalyst, isa method of determining the amount of reducing agent to be injected, orof decreasing the amount or release of unreacted reducing agent, bydetermining information as to the compositional content of the gasmixture after the gas mixture contacts a first catalyst but before thegas mixture contacts a second catalyst, and controlling the injection ofthe reducing agent in relation to the information as to thecompositional content of the gas mixture.

DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 depicts an array of chemo/electro-active materials.

[0014]FIG. 2 is a schematic of the pattern of interdigitated electrodesoverlaid with a dielectric overlayer, forming sixteen blank wells, in anarray of chemo/electro-active materials.

[0015]FIG. 3 depicts the electrode pattern, dielectric pattern, andsensor material pattern in an array of chemo/electro-active materials.

[0016]FIG. 4 is a schematic layout of the flow of a gas, such as thecombustion exhaust from a boiler, through an SCR system.

[0017]FIG. 5 is a schematic layout of the flow of a gas, such as thecombustion exhaust from a boiler, through an SCR system.

[0018]FIG. 6 shows the placement of a catalyst or a catalyst bed in anSCR system.

[0019]FIG. 7 is a schematic layout of the flow of a gas, such as thecombustion exhaust from a boiler, through an SCR system containing a gasanalyzer.

[0020]FIG. 8 is a schematic diagram of an internal combustion engineshowing the placement of a gas analyzer.

[0021]FIG. 9 is a schematic diagram of an internal combustion engineshowing the placement of a gas analyzer in connection with an SCRsystem.

DETAILED DESCRIPTION OF THE INVENTION

[0022] Nitrogen oxides may be reduced by contact with a reducing agentin the absence of a catalyst at a temperature of about 850 to about1200° C., preferably about 900 to about 1100° C. This is usuallyreferred to as selective non-catalytic reduction. The most common way ofproviding a temperature high enough to perform the reduction is toinject the reducing agent into the gas mixture that contains thenitrogen oxides in or near the source, such as a source of combustion,from which the nitrogen oxides are being emitted. The nitrogen oxidesare predominantly transformed by the high temperature of the source ofemissions to molecular nitrogen, which is nontoxic. Ammonia (e.g.anhydrous ammonia) is a reducing agent typically used, but urea is analternative choice as a reducing agent. Three to four times as muchreducing agent is required in a non-catalytic reduction, as compared toa catalytic reduction (described below), to achieve the same extent ofreduction.

[0023] More common, then, is selective catalytic reduction, in whichdiminution of the nitrogen oxide emitted by an emissions source, such asa source of combustion, takes place through contact of the nitrogenoxide and the reducing agent with a catalyst. In order to ensure anoptimal utilization of the needed reducing agent, selective catalyticreduction processes are preferred for the removal of nitrogen oxidesfrom emissions sources such as a combustion exhaust because of theoxygen content in the exhaust gas. As a reducing agent, ammonia gas(e.g. anhydrous ammonia) has proven itself to be suitable because itreacts easily with oxides of nitrogen in the presence of an appropriatecatalyst for the reaction, but only to a slight extent with the oxygenpresent in the gas. Urea is an alternative choice as a reducing agent.

[0024] For the selective reduction of the nitrogen oxides contained incombustion exhaust gases, for example, it is known to feed into theexhaust gas stream vaporous ammonia (NH₃) under pressure, or ammoniadissolved in water, without pressure, while an effort is made, by meansof a mixing section with appropriate baffling within the adjoiningconduit gas passages, to achieve a streamer-free distribution of ammoniaand temperatures in the flow of exhaust gas. The gas mixture emittedfrom a furnace flue may contain, for example, 1-20 percent by volume O₂,40 to 2000 ppm by volume nitrogen oxides, and 10 to 5000 ppm by volumeSO₂ and SO₃. The catalytic reduction of the nitrogen oxides by use ofammonia as a reducing agent typically proceeds according to one or moreof these reactions:

4NO+4NH₃+O₂→4N₂+6H₂O  I

2NO₂+4NH₃+O₂→3N₂+6H₂O  II

6NO+4NH₃→5N₂+6H₂O  III

6NO₂+8NH₃→7N₂+12H₂O  IV

NO+NO₂+2NH₃→2N₂+3H₂O  V

[0025] As shown in FIG. 4, in a typical combustion process, flue gasesemerging from a furnace (1) pass through a pipe (20) into a hotoperating electrofilter (2) where they are freed of dust. Anammonia/air-mixture is then introduced into contact with the gasesthrough injector (3), and is distributed homogeneously in the flow ofthe exhaust gas downstream from the filter (2). The mixture is then fedthrough pipe (22) into a catalytic reduction reactor (4).

[0026] It is shown in FIG. 4 that the catalyst (7) in the reactor (4)may be a vertical array of catalyst beds, a first series of beds (5)being positioned above a second series of beds (6). It is possible, ifdesired, to position a gas analyzer between individual catalyst beds, orbetween the first and second series of beds 5, 6. The catalyst may be inthe form, for example, of monolithic, ceramic honeycomb catalystsdisposed one behind the other to obtain the catalytic reduction ofnitrogen oxide in the exhaust gas. There is a broad range for thepermissible distances between the catalysts, or between the individualcatalyst beds, located in the reactor (4). The dimensions of the spacingarrangement of the catalysts or catalyst beds are determined to insurethe production of a turbulent transverse movement of gas in the conduitand avoidance of local mixing or “channeling”.

[0027] From the reduction reactor (4), the gas mixture may, if desired,be transported through pipe 24 to a sulfur oxide scrubber (8) whereinsulfur oxide is reacted with water or dilute aqueous sulfuric acid toform concentrated H₂SO₄. The completely purified exhaust gas leaving thescrubber (8) may then be transported by pipe (26) to chimney (9) fordischarge into the atmosphere. In FIG. 4, the exhaust is emitted fromits source, the furnace (1), and is transported through piping and othercomponents to its ultimate destination, the chimney (9), for dischargeinto the atmosphere. The direction of flow from the furnace (1) to thechimney (9) is considered to be downstream, and the opposite directionis considered to be upstream. The piping and other components throughwhich the exhaust gas mixture is transported, and in which the reactionof reduction occur, together provide a conduit for the flow, transport,handling and disposition of the gas mixture. A gas analyzer, or the gassensing component(s) thereof, can be positioned at any location alongthis conduit, whether in a pipe or within a component such as thecatalyst (7) located in reactor 4. Multiple catalyst beds areillustrated in the apparatus of FIG. 4, and in similar fashion, theapparatus may contain a plurality of catalysts as well.

[0028] Alternatively, as shown in FIG. 5, a dust filter (2) may belocated downstream from a catalyst (7). In a further alternative, asshown in FIG. 6, a gas mixture to be denitrified may pass horizontallythrough a reactor 30 containing one or more catalysts or catalyst beds.As described above, multiple catalysts and/or catalyst beds may beemployed in this horizontal configuration, and one or more gas analyzersmay be located between each of the catalysts and/or catalyst beds.

[0029] In the method according to the invention, essentially allcatalysts may be used which are suitable for the selective reduction ofnitrogen oxide. Examples of these are activated carbon, or catalyststhat are mixtures of the oxides of iron, titanium (e.g. amanganese-based TiO₂), tungsten, vanadium and molybdenum (see, forexample, DE 24 58 888, which is incorporated in its entirety as a parthereof for all purposes) or catalysts formed of natural or syntheticaluminum silicates, for example, zeolites (ZSM-5), or catalysts whichcontain precious metals of the platinum group. For example, a flue gasstream containing nitrogen oxides and sulphur oxides may be passedthrough a catalyst bed containing a catalyst consisting essentially of 3to 15% by weight vanadium pentoxide (V₂O₅) on a carrier consisting oftitanium dioxide (TiO₂), silica (SiO₂), and/or alumina (Al₂O₃).

[0030] The catalyst for nitrogen oxide reduction may be of anygeometrical shape, such as in the form of a honeycomb monolith or inpellet or particulate form. However, a catalyst shape resulting in alarge void and with parallel gas channels in the catalyst bed, such as ahoneycomb catalyst, is preferred since the conduit gas often containsconsiderable amounts of dust which otherwise might clog the catalystbed. The honeycomb form offers lower back pressure and a simplerpossibility for cleaning off dust. A denitrification catalyst could bemade for example as a carrier catalyst consisting of mullite honeycombbodies of the dimensions 150 mm×150 mm×150 mm length with a cell densityof 16/cm² and a zeolite coating of the mordenite type. A moving bed istypically used for granular activated carbon.

[0031] The catalyst can consist completely of a catalytically activemass (solid catalyst), or the catalytically active substance can bedeposited on an inert, ceramic or metallic body, which optionally can becoated in addition with a surface area enlarging oxide layer (carriercatalyst). For example, the catalyst may be in the form of a solid-bedreactor with a flow directed preferably vertically downward. The reactormay contain a honeycomb structure, which has a crystallinevanadium-titanium compound as the catalytically active substance. Thepressure loss in the solid-bed reactor is taken into account inestablishing the size of the conduit gas blower. The vertically downwardflow in the reactor is intended to combat the depositing of solidimpurities within the catalyst or keep them within acceptable ranges.The incrustation that occurs is removed discontinuously by blasting withcompressed air or steam.

[0032] The catalytic reaction, preferably carried out in a singlereactor, may be operated in the temperature range of about 250-550° C.,preferably about 350-450° C., and more preferably about 380-420° C. Thetemperature should not be so high that the reducing agent is degraded(as in the conversion, for example, of ammonia into NOx and water), orso low that the reducing agent does not fully react with the emittedNO_(x), is released into the atmosphere and becomes a pollutant itself.The molar ratio of reducing agent to nitrogen oxides is typically in therange of about 0.6-1.8, and preferably about 1.0-1.4. In the case of afull load operation in a facility containing a combustion source such asan electrical generating plant, a flue gas temperature of 350-400° C.may be easily reached, and these are temperatures at whichdenitrification catalysts can be utilized. In the case of a variableload operation, the flue gas temperature drops as a rule below theminimum required for the operation of the catalyst in the partial loadarea, so that a bypass connection system is typically necessary for thebranching off of flue gas before the last step of heat removal in theboiler in order to maintain the reaction temperature.

[0033] Operations that are carried out in the zone of high dust lead,moreover, to catalyst abrasion by the conduit dust, and may causedeposits and thus plugging up of the catalyst channels or pores. Toprevent such complications, a cleaning by blowing off with (for example)hot steam is required at relatively short time intervals. It ispreferred, however, that the reduction step be carried out using anexhaust gas which has little dust content or from which the dust hasbeen largely removed because the mechanical and thermal load of thecatalyst is considerably less. For the removal of the dust, the use of ahigh temperature electrofilter is particularly suitable. A filter ofthis type requires slightly higher investments in comparison to a coldoperating electrofilter, but reheating measures and problems which areconnected with the catalyst abrasion are avoided. Both embodiments inaddition have the advantage that the removal dust is not contaminatedwith reducing agent.

[0034] To obtain an efficient decrease in the content of the nitrogenoxides in a flue gas, one approach as noted above, has been to addreducing agent in excess of the stoichiometric amount needed accordingto reactions I-V. If the reducing agent is not completely converted inthe denitrification reaction, however, and a small quantity of it(designated as “ammonia slip” if the reducing agent is ammonia) ispresent in the exhaust gas after it is emitted into the atmosphere, theusual goal of limiting the content of reducing agent in treated flue gasto an acceptable level, such as 5-10 ppm by volume, will not be met. Thealternative of utilizing less than stoichiometric amounts of reducingagent, and compensating by the use of increased volumes of catalyst,will increase the catalyst costs. The efficiency of the denitrificationprocess will, moreover, be decreased as the absence of a stoichiometricamount of reducing agent will be the limiting factor in the reaction,and reduction of nitrogen oxides at an acceptable level will not occur.The methods and apparatus of this invention are used to furnishinformation about the compositional content of the gas mixture beingsubjected to denitrification to enable determination of the correctamount of reducing agent to be injected into the gas mixture, therebydecreasing the release of unreacted reducing agent.

[0035] For the purpose of controlling the denitrification reaction, itis also desirable to evaluate the success of the reaction by determininginformation about the compositional content of the gas mixture before itis emitted into the atmosphere. This type of determination may be made,for example, at one or more positions after the gas mixture has passedthe point of injection of the reducing agent, if the reaction isuncatalyzed, or after the gas mixture has passed downstream from areducing reactor if the reaction is catalyzed. Alternatively, if anoxidation catalyst is provided to oxidize unreacted reducing agent, thecompositionally-related information may be determined at one or morepositions after the gas mixture has passed downstream from the oxidationcatalyst.

[0036] When such an oxidation catalyst is employed, and the reducingagent is for example, ammonia, ammonia is oxidized to nitrogen and wateraccording to the following reaction:

4NH₃+3O₂→6H₂O+2N₂  VI

[0037] Typical oxidation catalysts for this purpose are based ontransition metals, for example those containing oxides of copper,chromium, manganese and/or iron. A catalyst consisting essentially ofabout 2 to 7% by weight vanadium promoted with at least one alkali metalin a vanadium to alkali metal atomic ratio in the range from about 1:2to about 1:5 on a silica carrier is advantageously employed since thiscatalyst gives a high degree of conversion according to the reaction VI.The alkali metal employed is preferably potassium.

[0038] One example of the manner in which the methods and apparatus ofthis invention can be used to control the reduction of a nitrogen oxideis to control the injection of the reducing agent into the nitrogenoxide, such as by controlling the injection of the reducing agent into agas mixture that contains a nitrogen oxide. In the case of nitrogenoxide that is emitted by a source of combustion, control of thereduction reaction may be effected in terms of the compositional contentof the stream of exhaust gas given off by the combustion. Informationmay be obtained that is related to the compositional content of theexhaust gas at points in time both before and after a reducing agent hasbeen injected into the nitrogen oxide.

[0039] Information related to the compositional content of a gas mixturecontaining a nitrogen oxide may be obtained from a gas analyzer that isexposed to the gas mixture. This is most conveniently done by placingone or more gas analyzers in a conduit in which the mixture containingthe nitrogen oxide is transported from its source of emission to itseventual destination, such as discharge into the atmosphere. In the caseof exhaust gas emitted from a source of combustion, this represents achallenge because combustion exhaust gases reach high temperatures thatwill degrade the materials and instrumentation from which manyanalytical devices are made. A gas analyzer as used in this invention isone that is not degraded by, or does not malfunction as a result ofexposure to, a gas or gas mixture having a temperature of about 300° C.or more. Preferably the analyzer is not degraded or does not malfunctionat even higher temperatures such as about 400° C. or more, about 500° C.or more, about 600° C. or more, about 700° C. or more, about 800° C. ormore, about 900° C. or more, or about 1000° C. or more. The gas analyzerused in this invention, including the reactive or gas sensing componentsthereof, may thus be positioned in a gas mixture having a temperature asdescribed above, and may thus be located in the same conduit in whichthe reducing agent is injected to effect the reduction reaction.Although the analyzer as it is installed In the conduit is connected toconductors that transmit signal outputs of the analyzer elsewhere forfurther processing, the only contact between the analyzer and thenitrogen oxide to be reduced, or the gas mixture containing the nitrogenoxides occurs in the conduit in which the nitrogen oxides aretransported from their source to their eventual destination. Theanalyzer is not operated by withdrawing gas from the conduit foranalysis in a separate chamber that is outside of the conduit.

[0040] A gas analyzer that is exposed to a gas mixture containing anitrogen oxide is used to provide information related to thecompositional content of the gas mixture for the purpose of controllingthe reduction reaction. The information is used, in particular tocontrol the injection of the reducing agent into the nitrogen oxide,such as by controlling the injection of the reducing agent into the gasmixture containing the nitrogen oxide. Information as to thecompositional content of the gas mixture obtained before the reducingagent has been injected, or before the gas mixture has contacted acatalyst (if a catalyst is used), may be used to assist in thecalculation of a stoichiometrically correct amount of reducing agent.This “stoichiometrically correct” amount is an amount that is sufficientto react with all nitrogen oxides present in the mixture withoutproviding an excess of reducing agent that will be transporteddownstream with the mixture as a pollutant itself. Information as to thecompositional content of the gas mixture obtained after the reducingagent has been injected may be used to evaluate the accuracy of thecalculation by which the stoichiometrically correct amount of reducingagent is determined. If it appears that the calculation is not accuratebecause the gas mixture downstream from the injector, and downstreamfrom the catalyst if a catalyst is used, contains more nitrogen oxidethan desired or more reducing agent than desired, adjustments can bemade to the calculation in view of such information obtained downstreamfrom the position of the reduction reaction.

[0041]FIG. 7 shows a schematic layout of one possible placement of a gasanalyzer both upstream 40 and downstream 42 from the position of areduction reactor 44 in which a catalyst is employed, also upstream 46from the point of injection of the reducing agent. By conductors 48, 50and 52, information about the compositional content of the gas mixtureis fed to a reducing agent control system 54. In addition to a pump forinjecting the reducing agent, the reducing agent control system maycontain a decision-making routine and/or a map. Information from gasanalyzer 46 may be fed forward to control system 54 to assist inperforming a first calculation of the amount of reducing agent to beinjected into the gas mixture. Information from gas analyzer 40 may befed back to control system 54 to evaluate whether the reducing agent isin place in the gas mixture to the extent and with the distribution asdesired, and, in view of such finding, to also assist in performingadjustments as needed on the original calculation of the amount ofreducing agent to be injected into the gas mixture. Information from gasanalyzer 42 may be fed back to control system 54 to evaluate whethernitrogen oxide and the reducing agent are both absent from the gasmixture to the extent desired, and, in view of such finding, to alsoassist in performing adjustments as needed on the original calculationof the amount of reducing agent to be injected into the gas mixture.

[0042] The gas source 56 could be a stationary source of combustion,such as a furnace or a boiler for a steam turbine; a source ofcombustion that can be stationary, mobile or self-propelled such as agas turbine or an internal combustion engine; or a chemical reactionthat does not involve combustion such as an industrial process. Althoughammonia is shown as the reducing agent, other reducing agents such asurea are also useful.

[0043] To control the operation of the reducing agent injector, thereducing agent control system performs certain decision-making routinesabout various operating characteristics of the reaction of reduction.The gas analyzers provide information to the control system aboutoperating characteristics such as the amount and rate of injection ofthe reducing agent, about the presence of the reducing agent in the gasmixture before the reaction occurs, and about the success of thereaction in terms of the extent of presence of nitrogen oxide and/orreducing agent in the gas mixture after the reaction is completed. Thereducing agent control system controls the injection of reducing agentby calculating an initial amount of reducing agent needed in view of theamount of nitrogen oxide determined to be present in the gas mixture,and by adjusting that calculation depending on the extent to which thereducing agent is successfully incorporated into the gas mixture beforethe reaction occurs, and depending on the extent to which nitrogen oxidehas been reacted out of the gas mixture without reducing agent slip.

[0044] The decision-making routine in the reducing agent control systemis run by a microprocessor chip, and applies one or more algorithmsand/or mathematical operations to that information to obtain a decisionin the form of a value that is equivalent to a desired state orcondition that should be possessed by a particular operatingcharacteristic. Based on the result of a decision-making routine,instructions are given by the reducing agent control system that cause achange in the rate or amount of injection of reducing agent thus movingthe reduction reaction as close as possible to ideal performance, whichis characterized by minimal residual nitrogen oxide and minimal reducingagent slip. In a preferred embodiment of this invention, a gas mixturethat contains a nitrogen oxide that is reduced is, after the reductionreaction, free or substantially free of nitrogen oxide, and/or is freeor substantially free of reducing agent.

[0045] In performing a decision-making routine, the reducing agentcontrol system may, and preferably does, employ a map. A map resides ina read-only memory, and is an electronic collection of information aboutvarious operating characteristics of the reaction of reduction. In oneembodiment, a range of quantified values may be set forth within the mapwith respect to a particular operating characteristic. This could be,for example, a range of temperature between 350 and 750° C., dividedinto 25° C. increments. With respect to each individual value of theparameter or operating characteristic in the range set forth, the mapmay then associate an acceptable value for one or more other operatingcharacteristics, or a factor to be used in a decision-making routine. Amap can be established in the form of a relational database, and can beaccessed by look-up instructions in a computer program.

[0046] In the performance of a decision-making routine to control theoperation of the reaction of reducing a nitrogen oxide, a value, such asthe size of an electrical signal, that is representative of the state orcondition of operating characteristic A may be inputed to the reducingagent control system. In one example of how the signal can then beutilized by a decision-making routine, the microprocessor chipdetermines a value representative of the state or condition each ofoperating characteristics B and C, and reads the map to determine, inview of the values for B and C, a target value D for operatingcharacteristic A. The target value could be a preselected value that isrecorded in the map as such, or could be a value that is calculated bythe reducing agent control system by a mathematical operation recordedin the map, with the calculation to specify D being made only on theoccasion when the values for B and C are determined. For example, adetermination may be made of the absolute value of the differencebetween A and B, and this absolute value, when added to C, becomes thetarget value D.

[0047] The value of operating characteristic A is compared to targetvalue D, and if A is in a desired relationship to D, the reducing agentcontrol system does not instruct that any adjustment in operations bemade. If A is not in a desired relationship to D, the decision-makingprocess could, in further alternative embodiments, read the map todetermine a desired value or range of values for A in terms of valuesfor operating characteristics E and F; or calculate a desired value forA by reading the map to determine coefficients to be used in performinga mathematical operation on E and F. The values for E and F could bedetermined at the time of making the decision, or could be preselectedvalues stored in the map. In either case, once the desired value for Ais determined, the reducing agent control system instructs the necessaryoperating characteristics of the reaction of reduction to be adjusted inthe manner necessary to obtain the desired value for A. This may be doneby adjusting operating characteristic A itself, or adjusting otheroperating characteristics that can influence the state or condition ofA. For example, the reaction of reduction may be controlled by adjustingthe amount or frequency of injection of reducing agent, by adjusting thetiming of injection by injectors in different locations, by heating orcooling the gas mixture or a reduction catalyst, and/or by adjusting theoperation of the emissions source such as by adjusting the fuel to airratio in a combustion reaction.

[0048] In this invention, information about the compositional content ofthe gas emitted by a chemical reaction, such as the exhaust gas of asource of combustion, may be used as an input to a decision-making inthe reducing agent control system. In the example described above,information about combustion exhaust gas could be used as therepresentative value that is inputed with respect to any one or more ofoperating characteristics A, B, C, E or F, or could be used as acoefficient in a operation that the decision-making routine causes to beperformed. Information about the gas composition is inputed to thedecision-making routine, in this invention, in the form of one or moresignals that is or are related to the individual concentration withinthe emitted gas stream of a particular individual component gas therein,or a particular subgroup of some but not all of the component gasestherein, or both an individual component and a subgroup. Therelationship may be a mathematical relationship, such as a monotonicrelationship, involving for example a log, inverse or scaled value. Thisis accomplished by exposing a gas analyzer, such as an array ofchemo/electro-active materials, to the emitted gas stream to generatethat may be, for example, an electrical or optical signal.

[0049] The ability to furnish information about the individualconcentration within an emitted gas stream of a particular component gasor subgroup therein makes it possible to calibrate a map. When buildinga map before a reaction or device to be controlled is put into service,values representative of a variety of parameters or operatingcharacteristics must be determined by systematically operating thereaction or device under a large enough sample of different conditionsto approximate all the conditions expected in actual service. A gasanalyzer, such as an array of chemo/electro-active materials, can beused to analyze the composition of the emitted gas stream to furnishinformation based on the concentration of individual components orsubgroups therein to be recorded in the map in relation to values ofother parameters or operating characteristics measured under the sameoperating conditions.

[0050] If preferred, however, this ability to furnish informationrelated to the concentration of individual components or subgroups in anemitted gas stream can be used to calibrate or re-calibrate a map inreal time while the reduction reaction is in service. For example, arelationship could be established in a map between a valuerepresentative of the concentration of an individual gas component orsubgroup, and values representative of various parameters or operatingcharacteristics, with the value for the gas concentration to be suppliedin real time. This might take the form of a decision-making routineinvolving a mathematical operation in which a value representative ofthe concentration of an individual gas component or subgroup is used asa factor or coefficient. The value representative of the concentrationof an individual gas component or subgroup could remain undetermineduntil the time that the mathematical operation is performed in theexecution of the decision-making routine to make the decision. The valuerepresentative of the concentration of an individual gas component orsubgroup is determined and supplied to the decision-making routine onlyon the occasion of making the decision, and the decision thus need notbe made based on information that may not be currently accurate at thetime the decision is made. A map in which one or more parameters oroperating characteristics is related to information about theconcentration of an individual gas component or subgroup, with theinformation about the gas concentration being furnished in real timewhile a reaction or device is in service, clearly then has substantialvalue because it is possible to essentially recalibrate the mapcontinually in real time.

[0051] In this invention, information about an emitted gas compositionmay be supplied to a map from a a gas analyzer employing one or morechemo/electro-active materials that furnishes an analysis of the emittedgas stream. Responses generated by the gas analyzer are then used asinputs, optionally along with the input from other sensors such as atemperature sensor, in the operation of algorithms that control thereaction of reduction.

[0052] In the case again of an engine, there are several ways in which agas analyzer, such as an apparatus containing one or morechemo/electro-active materials, can be incorporated into the operationof a reducing agent control system to control the injection of reducingagent and to control, ultimately, the reaction of reduction. Thechemo/elctro-active materials may be constructed as an array of sensorsthat have sensitivity to individual gaseous components or subgroups ofgases in a multi-component gas mixture, such as a stream of exhaust.Such sensors can be fabricated from semiconducting materials thatrespond uniquely to individual gases or gas subgroups that have commoncharacteristics such as similar oxidation potential, electronegativity,or ability to form free radicals. These are properties of interest whencharacterizing combustion.

[0053] Typical examples of individual gases and subgroups of gaseswithin an exhaust stream from a combustion reaction include oxygen,carbon monoxide, hydrogen, sulfur dioxide, ammonia, CO₂, H₂S, methanol,water, a hydrocarbon (such as C_(n)H_(2n+2), and as same may besaturated or unsaturated, or be optionally substituted with heteroatoms; and cyclic and aromatic analogs thereof), a nitrogen oxide (suchas NO, NO₂, N₂O or N₂O₄) or an oxygenated carbon (CO, CO₂ or C₅O₃). Theresponses of an array of chemo/electro-active materials to themulti-component mixture of such gases formed by a stream of exhaust canthus be used to determine what type of control over a reaction ofreduction is needed to execute a reaction in which nitrogen oxidecontent is decreased to the greatest extent possible without engenderingunacceptable reducing agent slip.

[0054] As an example, FIGS. 8 and 9 show several possible locations of agas analyzer, such as an array of sensor materials, in the exhaustsystem of a vehicular internal combustion engine. The engine in FIGS. 8and 9 contains a mass airflow and outside temperature sensor 60, an idleair valve 62, a throttle position valve 64, an exhaust gas recycle valve66, an air temperature sensor 68, a pressure sensor 70, an air intake72, an intake manifold 74, fuel injectors 76, spark plugs 78, a crankposition sensor 80, a cam position sensor 82, a coolant temperaturesensor 84, a pre-catalytic converter 86, an emissions control device(such as a catalytic converter and/or a device for the storage orabatement of NOx) 90, and a temperature sensor 92. The temperaturesensor shown in FIGS. 8 and 9 need not be located adjacent the emissionscontrol device 90 or the SCR catalyst 104, or additional temperaturesensors may be located elsewhere along the exhaust conduit. FIG. 8 showsthree possible locations 94, 96, 98 for a gas analyzer, which may beupstream or downstream from the emissions control device. The arrowsindicate the locations where it would be possible, if desired, toprovide for the flow of information to/from an engine control unitto/from one or more sensors or acctuators.

[0055] A gas analyzer at position 94 is located close to engine andresponds directly to the exhaust from individual cylinders. Because ofits proximity and fast response, the array in this location can be usedto obtain information from, or to control the operation of, eachindividual cylinder. An array in this location is exposed to very highexhaust temperatures for which semiconducting sensor materials are verysuitable. A gas sensor in position 96 in FIG. 8 operates cooler and isexposed to gasses that have already been modified in composition by theprecatalyst. However, the gas stream at this point still contains muchchemical information that can be used for control the reduction ofnitrogen oxides. This is also a suitable location to employ feed-forwardcontrol by using an array of sensor materials to control operation ofthe catalytic converter, which catalyzes the completion of the oxidationof unburned fuel. Position 98 is a location that can be used to monitorengine emissions and the current state of the catalytic converter. Basedon information from gas analyzer at this location, the catalyticconverter can be regenerated or otherwise controlled through feedbackprocess control.

[0056]FIG. 9 shows an SCR catalyst 104 and the deployment of gas sensorsin a control system in which a reducing agent is injected into theexhaust conduit at position 110. Reducing agent is supplied from areservoir 102 and is passed through reducing agent control system 100for injection into the exhaust conduit. Reducing agent control system100 includes the necessary pump to inject the reducing into the exhaustconduit, and is connected to the microprocessor chip for the passage ofsignals to and from the microprocessor chip to control the injection ofreducin agent. A gas analyzer, such as a gas sensor, can in thisarrangement be used either for feed-forward (position 106) or feedback(position 108) control. The gas sensor is responsive to a variety ofgases that may be present in a combustion exhaust stream such asammonia, nitrogen oxide, carbon monoxide, oxygen, hydrocarbons andwater. The reducing agent control system, and the injection of reducingagent, may be controlled by information obtained from a gas analyzerthat is positioned both upstream and/or downstream from a reductioncatalyst and, optionally, upstream and/or downstream from the reducingagent injector. Information about the compositional content of the gasmixture containing a nitrogen oxide is provided to a decision-makingroutine and/or map in the microprocessor chip for processing intosignals routed to the reducing agent pump, to the engine itself or toheating or cooling devices for the purpose of controlling the reactionof reduction.

[0057] An internal combustion engine, in which nitrogen oxide reductionis controlled by the methods and apparatus of this invention, can beused for many different purposes including, for example, in any type ofvehicle for transportation or recreation such as a car, truck, bus,locomotive, aircraft, spacecraft, boat, jet ski, all-terrain vehicle orsnowmobile; or in equipment for construction, maintenance or industrialoperations such as pumps, lifts, hoists, cranes, generators, orequipment for demolition, earth moving, digging, drilling, mining orgroundskeeping.

[0058] Although this invention has been described in detail with respectto the control of the reduction of nitrogen oxides generated bycombustion, i.e. the oxidation of a fossil fuel, it is equallyapplicable to the reduction of nitrogen oxides that may be found in agas mixture generated by any other type of chemical reaction. It is alsoequally applicable to the reduction of nitrogen oxides that are not in amixture with other gases, where, for example, a gas analyzer is used todetermine information related to the relative concentration within thegroup of nitrogen oxide of each individual nitrogen oxide. It is alsoequally applicable to reducing agents in addition to ammonia and urea.

[0059] It will thus be seen that, in various embodiments of thisinvention, as there may a plurality of reducing agent injectors, one ormore gas analyzers may be located in the conduit upstream or downstreamfrom each reducing agent injector. If a dust filter is used, it may belocated upstream from a reducing agent injector and/or one or more gasanalyzers.

[0060] If a catalyst is present, the catalyst may be located upstream ordownstream from one or more gas analyzers. A first catalyst may belocated upstream from one or more gas analyzers, and a second catalystmay be located downstream from one or more gas analyzers, particularlywhere the catalyst is a plurality of vertically arranged catalyst beds.A first gas analyzer may be located upstream from a catalyst, and asecond gas analyzer may be located downstream from the catalyst. One ormore gas analyzers may be located between first and second catalysts.One or more gas analyzers may be located at the point of destination ofa flowing stream of a gas mixture, such as at a point of discharge tothe atmosphere.

[0061] If a gas analyzer outputs a signal to a decision-making routine,a gas analyzer that is upstream from all catalyst, a gas analyzer thatis downstream from a first catalyst and upstream from a second catalyst,and/or a gas analyzer that is downstream from all catalyst may eachoutput a signal to a decision-making routine. A gas analyzer may outputat least one signal that is related to the individual concentrationwithin the gas mixture of an individual nitrogen oxide componenttherein, and/or may output at least one signal that is related to thecollective concentration within the gas mixture of all nitrogen oxidecomponents therein. The gas analyzer may in turn output a signal to amap. The gas analyzer may also output a signal to a decision-makingroutine that controls the injection of reducing agent, such as bycalculating an amount of reducing agent to be injected.

[0062] Information as to the compositional content of a gas mixture maybe determined before the injection of reducing agent, and/or before thegas mixture contacts any catalyst. Information as to the compositionalcontent of a gas mixture may also be determined after the gas mixturecontacts a first catalyst but before the gas mixture contacts a secondcatalyst, or after the gas mixture has contacted all catalyst. Forexample, a gas analyzer that is upstream from all catalyst, and a gasanalyzer that is downstream from all catalyst may each output separatesignals to a decision-making routine.

[0063] The injection of the reducing agent may be controlled in relationto such information as to the compositional content of the gas mixture,such as by determining the amount of reducing agent to be injected intothe gas mixture. The information as to the compositional content of thegas mixture may be an output of one or more gas analyzers, and may berelated to the individual concentration within the gas mixture of anindividual gas component therein (such as a nitrogen oxide), and/orrelated to the collective concentration within the gas mixture of asubgroup of the component gases therein (such as all nitrogen oxides).

[0064] In the present invention, an array of chemo/electro-activematerials is used for directly sensing one or more analyte gases in amulti-component gas system under variable temperature conditions. By“directly sensing” is meant that an array of gas-sensing materials willbe exposed to a mixture of gases that constitutes a multi-component gassystem, such as in a stream of flowing gases. The array may be situatedwithin the gas mixture, and more particularly within the source of thegas mixture, if desired. Alternatively, although not preferred, thearray may reside in a chamber to which the gas mixture is directed fromits source at another location. When gas is directed to a chamber inwhich an array is located, the gas mixture may be inserted in andremoved from the chamber by piping, conduits or any other suitable gastransmission equipment.

[0065] A response may be obtained upon exposure of the gas-sensingmaterials to the multi-component gas mixture, and the response will be afunction of the concentrations of one or more of the analyte gasesthemselves in the gas mixture. The sensor materials will be exposedsimultaneously (or substantially simultaneously) to each of the analytegases, and an analyte gas does not have to be physically separated fromthe multi-component gas mixture to be able to conduct an analysis of themixture and/or one or more analyte components thereof. This inventioncan be used, for example, to obtain responses to, and thus to detectand/or measure the concentrations, of combustion gases, such as oxygen,carbon monoxide, nitrogen oxides, hydrocarbons such as butane, CO₂, H₂S,sulfur dioxide, halogens, hydrogen, water vapor, an organo-phosphorusgas, and ammonia, at variable temperatures in gas mixtures such asautomobile emissions.

[0066] This invention utilizes an array of sensing materials to analyzea gas mixture and/or the components thereof to, for example, obtain aresponse to, detect the presence of and/or calculate the concentrationof one or more individual analyte gas components in the system. By“array” is meant at least two different materials that are spatiallyseparated, as shown for example in FIG. 1. The array may contain, forexample, 3, 4, 5, 6, 8, 10 or 12 gas-sensing materials, or other greateror lesser numbers as desired. It is preferred that there be provided atleast one sensor material for each of the individual gases or subgroupsof gases in the mixture to be analyzed. It may be desirable, however, toprovide more than one sensor material that is responsive to anindividual gas component and/or a particular subgroup of gases in themixture. For example, a group of at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 11or 12 sensors could be used to detect the presence of, and/or calculatethe concentration of, one or more individual component gases and/or oneor more subgroups of gases in the mixture. Groups of sensors, which mayor may not have members in common, could be used to obtain a response toan analyte that is an individual gas component or a subgroup of gases inthe mixture. A subgroup of gases that is, as the subgroup, an analytemay or may not contain as a member an individual gas that is itself alsoan analyte.

[0067] This invention is useful for detecting those gases that areexpected to be present in a gas stream. For example, in a combustionprocess, gases that are expected to be present include oxygen, nitrogenoxides (such as NO, NO₂, N₂O or N₂O₄), carbon monoxide, hydrocarbons(such as C_(n)H_(2n+2), and as same may be saturated or unsaturated, orbe optionally substituted with hetero atoms; and cyclic and aromaticanalogs thereof), ammonia or hydrogen sulfide, sulfur dioxide, CO₂, ormethanol. Other gases of interest may include alcohol vapors, solventvapors, hydrogen, water vapor, and those deriving from saturated andunsaturated hydrocarbons, ethers, ketones, aldehydes, carbonyls,biomolecules and microorganisms. The component of a multi-component gasmixture that is an analyte of interest may be an individual gas such ascarbon monoxide; may be a subgroup of some but not all of the gasescontained in the mixture, such as the nitrogen oxides (NO_(x)) orhydrocarbons; or may be a combination of one or more individual gasesand one or more subgroups. When a subgroup of gases is an analyte, achemo/electro-active material will respond to the collectiveconcentration within a multi-component gas mixture of the members of thesubgroup together.

[0068] The analyte gas(es) contained in the mixture to which thechemo/electro-active material will be exposed can be a single gas, asubgroup of gases together, or one or more gases or subgroups mixed withan inert gas such as nitrogen. Particular gases of interest are donorand acceptor gases. These are gases that either donate electrons to thesemiconducting material, such as carbon monoxide, H₂S and hydrocarbons,or accept electrons from the semiconducting material, such as O₂,nitrogen oxides (commonly depicted as NO_(x)), and halogens. Whenexposed to a donor gas, an n-type semiconducting material will have adecrease in electrical resistance, increasing the current, and it,therefore, will show an increase in temperature due to I²R heating. Whenexposed to an acceptor gas, an n-type semiconducting material will havean increase in electrical resistance, decreasing the current, andtherefore will show a decrease in temperature due to I²R heating. Theopposite occurs in each instance with p-type semiconducting materials.

[0069] Obtaining information related to the compositional content of agas mixture using these sensor materials, such as measurement of gasconcentrations, can be based on a change in an electrical property, suchas AC impedance, of at least one, but preferably each and all, of thematerials upon exposure of the materials to a mixture containing one ormore analyte gases. Analysis of a gas mixture can also be performed interms of extent of change in other electrical properties of the sensormaterials, such as capacitance, voltage, current or AC or DC resistance.Change in DC resistance may be determined, for example, by measuringchange in temperature at constant voltage. The change in one of theseillustrative properties of a sensor material is a function of thepartial pressure of an analyte gas within the gas mixture, which in turndetermines the concentration at which the molecules of the analyte gasesbecome adsorbed on the surface of a sensor material, thus affecting theelectrical response characteristics of that material. By using an arrayof chemo/electro-active materials, a pattern of the respective responsesexhibited by the materials upon exposure to a mixture containing one ormore analyte gases can be used to simultaneously and directly detect thepresence of, and/or measure the concentration of, at least one gas in amulti-component gas system. The invention, in turn, can be used todetermine the composition of the gas system. The concept is illustratedschematically in FIG. 1 and is exemplified below.

[0070] To illustrate, consider the theoretical example below of theexposure of a sensor material to a mixture containing an analyte gas.Where a response is obtained, the event is depicted as positive (+), andwhere no response is obtained, the event is depicted as negative (−).Material 1 responds to Gas 1 and Gas 2, but shows no response to Gas 3.Material 2 responds to Gas 1 and Gas 3, but shows no response to Gas 2,and Material 3 responds to Gas 2 and Gas 3, but shows no response toGas 1. Material 1 Material 2 Material 3 Gas 1 + + − Gas 2 + − + Gas 3− + +

[0071] Therefore, if an array consisting of Materials 1, 2 and 3 givesthe following response to an unknown gas, Material 1 Material 2 Material3 Unknown Gas + − +

[0072] then the unknown gas would be identified as Gas 2. The responseof each sensor material would be a function of the partial pressurewithin the mixture of, and thus the concentration of, an analyte gas orthe collective concentration of a subgroup of analyte gases; and theresponse could be quantified or recorded as a processible value, such asa numerical value. In such case, the values of one or more responses canbe used to generate quantitative information about the presence withinthe mixture of one or more analyte gases. In a multicomponent gassystem, chemometrics, neural networks or other pattern recognitiontechniques could be used to calculate the concentration of one or moreanalyte gases in the mixture of the system.

[0073] The sensing materials used are chemo/electro-active materials. A“chemo/electro-active material” is a material that has an electricalresponse to at least one individual gas in a mixture. Some metal oxidesemiconducting materials, mixtures thereof, or mixtures of metal oxidesemiconductors with other inorganic compounds are chemo/electro-active,and are particularly useful in this invention. Each of the variouschemo/electro-active materials used herein preferably exhibits anelectrically detectable response of a different kind and/or extent, uponexposure to the mixture and/or an analyte gas, than each of the otherchemo/electro-active materials. As a result, an array of appropriatelychosen chemo/electro-active materials can be used to analyze amulti-component gas mixture, such as by interacting with an analyte gas,sensing an analyte gas, or determining the presence and/or concentrationof one or more analyte gases or subgroups in a mixture, despite thepresence therein of interfering gases that are not of interest.Preferably the mole percentages of the major components of eachgas-sensing material differs from that of each of the others.

[0074] The chemo/electro-active material can be of any type, butespecially useful are semiconducting metal oxides such as SnO₂, TiO₂,WO₃ and ZnO. These particular materials are advantageous due to theirchemical and thermal stability. The chemo/electro-active material can bea mixture of two or more semiconducting materials, or a mixture of asemiconducting material with an inorganic material, or combinationsthereof. The semiconducting materials of interest can be deposited on asuitable solid substrate that is an insulator such as, but not limitedto, alumina or silica and is stable under the conditions of themulti-component gas mixture. The array then takes the form of the sensormaterials as deposited on the substrate. Other suitable sensor materialsinclude single crystal or polycrystalline semiconductors of the bulk orthin film type, amorphous semiconducting materials, and semiconductormaterials that are not composed of metal oxides.

[0075] The chemo/electro-active materials that contain more than onemetal do not have to be a compound or solid solution, but can be amulti-phase physical mixture of discrete metals and/or metal oxides. Asthere will be varying degrees of solid state diffusion by the precursormaterials from which the chemo/electro-active materials are formed, thefinal materials may exhibit composition gradients, and they can becrystalline or amorphous. Suitable metal oxides are those that

[0076] i) when at a temperature of about 400° C. or above, have aresistivity of about 1 to about 106 ohm-cm, preferably about 1 to about105 ohm-cm, and more preferably about 10 to about 104 ohm-cm;

[0077] ii) show a chemo/electro response to at least one gas ofinterest; and

[0078] iii) are stable and have mechanical integrity, that is are ableto adhere to the substrate and not degrade at the operating temperature.

[0079] The metal oxides may also contain minor or trace amounts ofhydration and elements present in the precursor materials.

[0080] The sensor materials may optionally contain one or more additivesto promote adhesion to a substrate, or that alter the conductance,resistance or selectivity of the sensor material. Examples of additivesto alter the conductance, resistance or selectivity of the sensormaterial include Ag, Au or Pt, as well as frits. Examples of additivesto promote adhesion include frits, which are finely ground inorganicminerals that are transformed into glass or enamel on heating, or arapidly quenched glass that retains its amorphous quality in the solidstate. Frit percursor compounds are melted at high temperature andquenched, usually by rapidly pouring the melt into a fluid such aswater, or by pouring through spinning metal rollers. The precursorcompounds usually are a mechanical mixture of solid compounds such asoxides, nitrates or carbonates, or can be co-precipitated or gelled froma solution. Suitable precursor materials for frits include alkali andalkaline earth alumino-silicates and alumino-boro-silicates, copper,lead, phosphorus, titanium, zinc and zirconium. Frits as additives maybe used in amounts of up to 30 volume percent, and preferably up to 10volume percent, of the total volume of the chemo/electro-active materialfrom which the sensor is made.

[0081] If desired, the sensor materials may also contain additives that,for example, catalyze the oxidation of a gas of interest or promote theselectivity for a particular analyte gas; or contain one or more dopantsthat convert an n semiconductor to a p semiconductor, or vice versa.These additives may be used in amounts of up to 30 weight percent, andpreferably up to 10 weight percent, of the chemo/electro-active materialfrom which the sensor is made.

[0082] Any frits or other additives used need not be uniformly orhomogeneously distributed throughout the sensor material as fabricated,but may be localized on or near a particular surface thereof as desired.Each chemo/electro-active material may, if desired, be covered with aporous dielectric overlayer.

[0083] The chemo/electro-active materials used as sensor materials inthis invention may, for example, be metal oxides of the formula M¹O_(x),M¹ _(a)M² _(b)O_(x), or M¹ _(a)M² _(b)M³ _(c)O_(x); or mixtures thereof,wherein

[0084] M¹, M² and M³ are metals that form stable oxides when fired inthe presence of oxygen above 500° C.;

[0085] M¹ is selected from Periodic Groups 2-15 and the lanthanidegroup;

[0086] M² and M³ are each independently selected from Periodic Groups1-15, and the lanthanide group;

[0087] M¹ and M² are not the same in M¹ _(a)M² _(b)O_(x), and M¹, M² andM³ are not the same in M¹ _(a)M² _(b)M³ _(c)O_(x);

[0088] a, b, and c are each independently in the range of about 0.0005to about 1; and

[0089] x is a number sufficient so that the oxygen present balances thecharges of the other elements present in the chemo/elelctro-activematerial.

[0090] In certain preferred embodiments, the metal oxide materials mayinclude those in which

[0091] M¹ is selected from the group consisting of Ce, Co, Cu, Fe, Ga,Nb, Ni, Pr, Ru, Sn, Ti, Tm, W, Yb, Zn, and Zr; and/or

[0092] M² and M³ are each independently selected from the groupconsisting of Al, Ba, Bi, Ca, Cd, Ce, Co, Cr, Cu, Fe, Ga, Ge, In, K, La,Mg, Mn, Mo, Na, Nb, Ni, Pb, Pr, Rb, Ru, Sb, Sc, Si, Sn, Sr, Ta, Ti, Tm,V, W, Y, Yb, Zn, and Zr;

[0093] but in which M¹ and M² are not the same in M¹ _(a)M² _(b)O_(x),and M¹, M² and M³ are not the same in M¹ _(a)M² _(b)M³ _(c)O_(x).

[0094] In certain other preferred embodiments, the metal oxide materialsmay include those in which

[0095] M¹O_(x) is CeO_(x), CoO_(x), CuO_(x), FeO_(x), NaO_(x), NbO_(x),NiO_(x), PrO_(x), RuO_(x), SnO_(x), TaO_(x), TiO_(x), TmO_(x), WO_(x),YbO_(x), ZnO_(x), ZrO_(x), SnO_(x) with Ag additive, ZnO_(x) with Agadditive, TiO_(x) with Pt additive, ZnO_(x) with frit additive, NiO_(x)with frit additive, SnO_(x) with frit additive, or WO_(x) with fritadditive; and/or

[0096] M¹ _(a)M² _(b)O_(x) is Al_(a)Cr_(b)O_(x), Al_(a)Fe_(b)O_(x),Al_(a)Mg_(b)O_(x), Al_(a)Ni_(b)O_(x), Al_(a)Ti_(b)O_(x),Al_(a)V_(b)O_(x), Ba_(a)Cu_(b)O_(x), Ba_(a)Sn_(b)O_(x),Ba_(a)Zn_(b)O_(x), Bi_(a)Ru_(b)O_(x), Bi_(a)Sn_(b)O_(x),Bi_(a)Zn_(b)O_(x), Ca_(a)Sn_(b)O_(x), Ca_(a)Zn_(b)O_(x),Cd_(a)Sn_(b)O_(x), Cd_(a)Zn_(b)O_(x), Ce_(a)Fe_(b)O_(x),Ce_(a)Nb_(b)O_(x), Ce_(a)Ti_(b)O_(x), Ce_(a)V_(b)O_(x),Co_(a)Cu_(b)O_(x), Co_(a)Ge_(b)O_(x), Co_(a)La_(b)O_(x),Co_(a)Mg_(b)O_(x), Co_(a)Nb_(b)O_(x), Co_(a)Pb_(b)O_(x),Co_(a)Sn_(b)O_(x), Co_(a)V_(b)O_(x), Co_(a)W_(b)O_(x),Co_(a)Zn_(b)O_(x), Cr_(a)Cu_(b)O_(x), Cr_(a)La_(b)O_(x),Cr_(a)Mn_(b)O_(x), Cr_(a)Ni_(b)O_(x), Cr_(a)Si_(b)O_(x),Cr_(a)Ti_(b)O_(x), Cr_(a)Y_(b)O_(x), Cr_(a)Zn_(b)O_(x),Cu_(a)Fe_(b)O_(x), Cu_(a)Ga_(b)O_(x), Cu_(a)La_(b)O_(x),Cu_(a)Na_(b)O_(x), Cu_(a)Ni_(b)O_(x), Cu_(a)Pb_(b)O_(x),Cu_(a)Sn_(b)O_(x), Cu_(a)Sr_(b)O_(x), Cu_(a)Ti_(b)O_(x),Cu_(a)Zn_(b)O_(x), Cu_(a)Zr_(b)O_(x), Fe_(a)Ga_(b)O_(x),Fe_(a)La_(b)O_(x), Fe_(a)Mo_(b)O_(x), Fe_(a)Nb_(b)O_(x),Fe_(a)Ni_(b)O_(x), Fe_(a)Sn_(b)O_(x), Fe_(a)Ti_(b)O_(x),Fe_(a)W_(b)O_(x), Fe_(a)Zn_(b)O_(x), Fe_(a)Zr_(b)O_(x),Na_(a)La_(b)O_(x), Na_(a)Sn_(b)O_(x), Ne_(a)Nb_(b)O_(x),Ne_(a)Ti_(b)O_(x), In_(a)Sn_(b)O_(x), K_(a)Nb_(b)O_(x),Mn_(a)Nb_(b)O_(x), Mn_(a)Sn_(b)O_(x), Mn_(a)Ti_(b)O_(x),Mn_(a)Y_(b)O_(x), Mn_(a)Zn_(b)O_(x), Mn_(a)Pb_(b)O_(x),Mo_(a)Rb_(b)O_(x), Mo_(a)Sn_(b)O_(x), Mo_(a)Ti_(b)O_(x),Mo_(a)Zn_(b)O_(x), Nb_(a)Ni_(b)O_(x), Nb_(a)Ni_(b)O_(x),Nb_(a)Sr_(b)O_(x), Nb_(a)Ti_(b)O_(x), Nb_(a)W_(b)O_(x),Nb_(a)Zr_(b)O_(x), Ni_(a)Si_(b)O_(x), Ni_(a)Sn_(b)O_(x),Ni_(a)Y_(b)O_(x), Ni_(a)Zn_(b)O_(x), Ni_(a)Zr_(b)O_(x),Pb_(a)Sn_(b)O_(x), Pb_(a)Zn_(b)O_(x), Rb_(a)W_(b)O_(x),Ru_(a)Sn_(b)O_(x), Ru_(a)W_(b)O_(x), Ru_(a)Zn_(b)O_(x),Sb_(a)Sn_(b)O_(x), Sb_(a)Zn_(b)O_(x), Sc_(a)Zr_(b)O_(x),Si_(a)Sn_(b)O_(x), Si_(a)Ti_(b)O_(x), Si_(a)W_(b)O_(x),Si_(a)Zn_(b)O_(x), Sn_(a)Ta_(b)O_(x), Sn_(a)Ti_(b)O_(x),Sn_(a)W_(b)O_(x), Sn_(a)Zn_(b)O_(x), Sn_(a)Zr_(b)O_(x),Sr_(a)Ti_(b)O_(x), Ta_(a)Ti_(b)O_(x), Ta_(a)Zn_(b)O_(x),Ta_(a)Zr_(b)O_(x), Ti_(a)V_(b)O_(x), Ti_(a)W_(b)O_(x),Ti_(a)Zn_(b)O_(x), Ti_(a)Zr_(b)O_(x), V_(a)Zn_(b)O_(x),V_(a)Zr_(b)O_(x), W_(a)Zn_(b)O_(x), W_(a)Zr_(b)O_(x), Y_(a)Zr_(b)O_(x),Zn_(a)Zr_(b)O_(x), Al_(a)Ni_(b)O_(x) with frit additive,Cr_(a)Ti_(b)O_(x) with frit additive, Fe_(a)La_(b)O_(x) with fritadditive, Fe_(a)Ni_(b)O_(x) with frit additive, Fe_(a)Ti_(b)O_(x) withfrit additive, Nb_(a)Ti_(b)O_(x) with frit additive, Nb_(a)W_(b)O_(x)with frit additive, Ni_(a)Zn_(b)O_(x) with frit additive,Ni_(a)Zr_(b)O_(x) with frit additive, Sb_(a)Sn_(b)O_(x) with fritadditive, Ta_(a)Ti_(b)O_(x) with frit additive, or Ti_(a)Zn_(b)O_(x)with frit additive; and/or

[0097] M¹ _(a)M² _(b)M³ _(c)O_(x) is Al_(a)Mg_(b)Zn_(c)O_(x),Al_(a)Si_(b)V_(c)O_(x), Ba_(a)Cu_(b)Ti_(c)O_(x),Ca_(a)Ce_(b)Zr_(c)O_(x), Co_(a)Ni_(b)Ti_(c)O_(x),Co_(a)Ni_(b)Zr_(c)O_(x), Co_(a)Pb_(b)Sn_(c)O_(x),Co_(a)Pb_(b)Zn_(c)O_(x), Cr_(a)Sr_(b)Ti_(c)O_(x),Cu_(a)Fe_(b)Mn_(c)O_(x), Cu_(a)La_(b)Sr_(c)O_(x),Fe_(a)Nb_(b)Ti_(c)O_(x), Fe_(a)Pb_(b)Zn_(c)O_(x),Fe_(a)Sr_(b)Ti_(c)O_(x), Fe_(a)Ta_(b)Ti_(c)O_(x),Fe_(a)W_(b)Zr_(c)O_(x), Ga_(a)Ti_(b)Zn_(c)O_(x),La_(a)Mn_(b)Na_(c)O_(x), La_(a)Mn_(b)Sr_(c)O_(x),Mn_(a)Sr_(b)Ti_(c)O_(x), Mo_(a)Pb_(b)Zn_(c)O_(x),Nb_(a)Sr_(b)Ti_(c)O_(x), Nb_(a)Sr_(b)W_(c)O_(x),Nb_(a)Ti_(b)Zn_(c)O_(x), Ni_(a)Sr_(b)Ti_(c)O_(x),Sn_(a)W_(b)Zn_(c)O_(x), Sr_(a)Ti_(b)V_(c)O_(x), Sr_(a)Ti_(b)Zn_(c)O_(x),or Ti_(a)W_(b)Zr_(c)O_(x).

[0098] In certain other preferred embodiments, the metal oxide materialsmay include those that are in an array of first and secondchemo/electro-active materials, wherein the chemo/electro-activematerials are selected from the pairings in the group consisting of

[0099] (i) the first material is M¹O_(x), and the second material is M¹_(a)M² _(b)O_(x);

[0100] (ii) the first material is M¹O_(x), and the second material is M¹_(a)M² _(b)M³ _(c)O_(x);

[0101] (iii) the first material is M¹ _(a)M² _(b)O_(x), and the secondmaterial is M¹ _(a)M² _(b)M³ _(c)O_(x);

[0102] (iv) the first material is a first M¹O_(x), and the secondmaterial is a second M¹O_(x);

[0103] (v) the first material is a first M¹ _(a)M² _(b)O_(x) and thesecond material is a second M¹ _(a)M² _(b)O_(x); and

[0104] (vi) the first material is a first M¹ _(a)M² _(b)M³ _(c)O_(x),and the second material is a second M¹ _(a)M² _(b)M³ _(c)O_(x);

[0105] wherein

[0106] M¹ is selected from the group consisting of Ce, Co, Cu, Fe, Ga,Nb, Ni, Pr, Ru, Sn, Ti, Tm, W, Yb, Zn, and Zr;

[0107] M² and M³ are each independently selected from the groupconsisting of Al, Ba, Bi, Ca, Cd, Ce, Co, Cr, Cu, Fe, Ga, Ge, In, K, La,Mg, Mn, Mo, Na, Nb, Ni, Pb, Pr, Rb, Ru, Sb, Sc, Si, Sn, Sr, Ta, Ti, Tm,V, W, Y, Yb, Zn, and Zr;

[0108] but M¹ and M² are not the same in M¹ _(a)M² _(b)O_(x) and M¹, M²and M³ are not the same in M¹ _(a)M² _(b)M³ _(c)O_(x);

[0109] a, b and c are each independently about 0.0005 to about 1; and

[0110] x is a number sufficient so that the oxygen present balances thecharges of the other elements present in the chemo/electro-activematerial.

[0111] In certain other preferred embodiments, an array of two or morechemo/electro-active materials may be selected from the group consistingof (i) the chemo/electro-active materials that include M¹O_(x), (ii) thechemo/electro-active materials that include M¹ _(a)M² _(b)O_(x), and(iii) the chemo/electro-active materials that include M¹ _(a)M² _(b)M³_(c)O_(x);

[0112] wherein M¹ is selected from the group consisting of Al, Ce, Cr,Cu, Fe, Ga, Mn, Nb, Ni, Pr, Sb, Sn, Ta, Ti, W and Zn;

[0113] wherein M² and M³ are each independently selected from the groupconsisting of Ga, La, Mn, Ni, Sn, Sr, Ti, W, Y, Zn;

[0114] wherein M¹ and M² are each different in M¹ _(a)M² _(b)O_(x), andM¹, M² and M³ are each different in M¹ _(a)M² _(b)M³ _(c)O_(x);

[0115] wherein a, b and c are each independently about 0.0005 to about1; and

[0116] wherein x is a number sufficient so that the oxygen presentbalances the charges of the other elements in the chemo/electro-activematerial.

[0117] M¹ may for example be selected from the group consisting of Al,Cr, Fe, Ga, Mn, Nb, Ni, Sb, Sn, Ta, Ti and Zn, or from the groupconsisting of Ga, Nb, Ni, Sb, Sn, Ta, Ti and Zn. M², M³, or M² and M³may be selected from the group consisting of La, Ni, Sn, Ti and Zn, orthe group consisting of Sn, Ti and Zn.

[0118] The array may contain other numbers of chemo/electro-activematerials such as four or eight, and the array may contain at least onechemo/electro-active material that comprises M¹O_(x), and at least threechemo/electro-active materials that each comprise M1aM2bOx.Alternatively, the array may contain (i) at least onechemo/electro-active material that comprises M¹O_(x), and at least fourchemo/electro-active materials that each comprise M1aM2bOx; or (ii) atleast two chemo/electro-active materials that each comprise M1Ox, and atleast four chemo/electro-active materials that each comprise M1aM2bOx;or (iii) at least three chemo/electro-active materials that eachcomprise M1aM2bOx and at least one chemo/electro-active material thatcomprises M1aM2bM3cOx.

[0119] Chemo/electro-active materials useful in the apparatus of thisinvention may be selected from one or more members of the groupconsisting of

[0120] a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)

[0121] a chemo/electro-active material that comprises CeO₂,

[0122] a chemo/electro-active material that comprises Cr_(a)Mn_(b)O_(x),

[0123] a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)

[0124] a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x)

[0125] a chemo/electro-active material that comprises Cu_(a)Ga_(b)O_(x),

[0126] a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x)

[0127] a chemo/electro-active material that comprises CuO,

[0128] a chemo/electro-active material that comprises Fe_(a)La_(b)O_(x)

[0129] a chemo/electro-active material that comprises Fe_(a)Ni_(b)O_(x)

[0130] a chemo/electro-active material that comprises Fe_(a)Ti_(b)O_(x)

[0131] a chemo/electro-active material that comprisesGa_(a)Ti_(b)Zn_(c)O_(x)

[0132] a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x)

[0133] a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x),

[0134] a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)

[0135] a chemo/electro-active material that comprisesNb_(a)Ti_(b)Zn_(c)O_(x)

[0136] a chemo/electro-active material that comprises Nb_(a)W_(b)O_(x)

[0137] a chemo/electro-active material that comprises NiO,

[0138] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)

[0139] a chemo/electro-active material that comprises Pr₆O₁₁,

[0140] a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x).

[0141] a chemo/electro-active material that comprises SnO₂,

[0142] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x),and a chemo/electro-active material that comprises Ti_(a)Zn_(b)O_(x).

[0143] a chemo/electro-active material that comprises WO₃, and

[0144] a chemo/electro-active material that comprises ZnO.

[0145] wherein a, b and c are each independently about 0.0005 to about1; and wherein x is a number sufficient so that the oxygen presentbalances the charges of the other elements in the chemo/electro-activematerial.

[0146] Chemo/electro-active materials useful in this invention may alsobe selected from subgroups of the foregoing formed by omitting any oneor more members from the whole group as set forth in the list above. Asa result, the chemo/electro-active materials may in such instance notonly be any one or more member(s) selected from any subgroup of any sizethat may be formed from the whole group as set forth in the list above,but the subgroup may also exclude the members that have been omittedfrom the whole group to form the subgroup. The subgroup formed byomitting various members from the whole group in the list above may,moreover, contain any number of the members of the whole group such thatthose members of the whole group that are excluded to form the subgroupare absent from the subgroup. Representative subgroups are set forthbelow.

[0147] Chemo/electro-active materials that comprise M1Ox may, forexample, be selected from the group consisting of

[0148] a chemo/electro-active material that comprises CeO₂,

[0149] a chemo/electro-active material that comprises CuO,

[0150] a chemo/electro-active material that comprises NiO,

[0151] a chemo/electro-active material that comprises Pr₆O₁₁,

[0152] a chemo/electro-active material that comprises SnO₂,

[0153] a chemo/electro-active material that comprises WO₃, and

[0154] a chemo/electro-active material that comprises ZnO.

[0155] Of the above, one or more members of the group consisting of

[0156] a chemo/electro-active material that comprises CeO₂,

[0157] a chemo/electro-active material that comprises SnO₂, and

[0158] a chemo/electro-active material that comprises ZnO

[0159] may contain a frit additive.

[0160] A chemo/electro-active material that comprises M1aM2bOx or achemo/electro-active material that comprises M1aM2bM3cOx may be selectedfrom the group consisting of

[0161] a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)

[0162] a chemo/electro-active material that comprises Cr_(a)Mn_(b)O_(x),

[0163] a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)

[0164] a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x)

[0165] a chemo/electro-active material that comprises Cu_(a)Ga_(b)O_(x),

[0166] a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x)

[0167] a chemo/electro-active material that comprises Fe_(a)La_(b)O_(x)

[0168] a chemo/electro-active material that comprises Fe_(a)Ni_(b)O_(x)

[0169] a chemo/electro-active material that comprises Fe_(a)Ti_(b)O_(x)

[0170] a chemo/electro-active material that comprisesGa_(a)Ti_(b)Zn_(c)O_(x)

[0171] a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x)

[0172] a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x),

[0173] a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)

[0174] a chemo/electro-active material that comprisesNb_(a)Ti_(b)Zn_(c)O_(x)

[0175] a chemo/electro-active material that comprises Nb_(a)W_(b)O_(x)

[0176] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)

[0177] a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x).

[0178] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x),and

[0179] a chemo/electro-active material that comprises Ti_(a)Zn_(b)O_(x).

[0180] Of the above, one or more members of the group consisting of

[0181] a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)

[0182] a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)

[0183] a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x)

[0184] a chemo/electro-active material that comprises Fe_(a)La_(b)O_(x)

[0185] a chemo/electro-active material that comprises Fe_(a)Ni_(b)O_(x)

[0186] a chemo/electro-active material that comprises Fe_(a)Ti_(b)O_(x)

[0187] a chemo/electro-active material that comprisesGa_(a)Ti_(b)Zn_(c)O_(x)

[0188] a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)

[0189] a chemo/electro-active material that comprisesNb_(a)Ti_(b)Zn_(c)O_(x)

[0190] a chemo/electro-active material that comprises Nb_(a)W_(b)O_(x)

[0191] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)

[0192] a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x)

[0193] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x),and

[0194] a chemo/electro-active material that comprises Ti_(a)Zn_(b)O_(x)

[0195] may contain a frit additive.

[0196] In the apparatus of this invention, a chemo/electro-activematerial that comprises M¹ _(a)M² _(b)O_(x) may be selected from thegroup consisting of

[0197] a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)

[0198] a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x),and

[0199] a chemo/electro-active material that comprises Fe_(a)La_(b)O_(x).

[0200] or the group consisting of

[0201] a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)

[0202] a chemo/electro-active material that comprises Fe_(a)La_(b)O_(x),and

[0203] a chemo/electro-active material that comprises Fe_(a)Ni_(b)O_(x)

[0204] or the group consisting of

[0205] a chemo/electro-active material that comprises Fe_(a)La_(b)O_(x)

[0206] a chemo/electro-active material that comprises Fe_(a)Ni_(b)O_(x),and

[0207] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)

[0208] or the group consisting of

[0209] a chemo/electro-active material that comprises Fe_(a)Ni_(b)O_(x)

[0210] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x),and

[0211] a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x).

[0212] or the group consisting of

[0213] a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)

[0214] a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)

[0215] a chemo/electro-active material that comprises Fe_(a)La_(b)O_(x)

[0216] a chemo/electro-active material that comprises Fe_(a)Ni_(b)O_(x)

[0217] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x),and

[0218] a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x).

[0219] or the group consisting of

[0220] a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)

[0221] a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x),and

[0222] a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x)

[0223] or the group consisting of

[0224] a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)

[0225] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x),and

[0226] a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x)

[0227] or the group consisting of

[0228] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)

[0229] a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x),and

[0230] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x)

[0231] or the group consisting of

[0232] a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x)

[0233] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x),and

[0234] a chemo/electro-active material that comprises Ti_(a)Zn_(b)O_(x).

[0235] or the group consisting of

[0236] a chemo/electro-active material that comprises Cr_(a)Mn_(b)O_(x)

[0237] a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x),and

[0238] a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x)

[0239] or the group consisting of

[0240] a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)

[0241] a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x),and

[0242] a chemo/electro-active material that comprises Cu_(a)Ga_(b)O_(x)

[0243] or the group consisting of

[0244] a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x)

[0245] a chemo/electro-active material that comprises Cu_(a)Ga_(b)O_(x),and

[0246] a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x)

[0247] or the group consisting of

[0248] a chemo/electro-active material that comprises Cu_(a)Ga_(b)O_(x)

[0249] a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x),and

[0250] a chemo/electro-active material that comprises Fe_(a)La_(b)O_(x).

[0251] or the group consisting of

[0252] a chemo/electro-active material that comprises Cr_(a)Mn_(b)O_(x)

[0253] a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)

[0254] a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x)

[0255] a chemo/electro-active material that comprises Cu_(a)Ga_(b)O_(x)

[0256] a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x),and

[0257] a chemo/electro-active material that comprises Fe_(a)La_(b)O_(x).

[0258] or the group consisting of

[0259] a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x)

[0260] a chemo/electro-active material that comprises Cu_(a)Ga_(b)O_(x),and

[0261] a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x)

[0262] or the group consisting of

[0263] a chemo/electro-active material that comprises Cu_(a)Ga_(b)O_(x),

[0264] a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x),and

[0265] a chemo/electro-active material that comprises Fe_(a)Ti_(b)O_(x)

[0266] or the group consisting of

[0267] a chemo/electro-active material that comprises Cr_(a)Mn_(b)O_(x)

[0268] a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x),and

[0269] a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x)

[0270] In the apparatus of this invention, a chemo/electro-activematerial that comprises M1aM2bOx, or a chemo/electro-active materialthat comprises M1aM2bM3cOx may be selected from the group consisting of

[0271] a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)

[0272] a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x),and

[0273] a chemo/electro-active material that comprisesNb_(a)Ti_(b)Zn_(c)O_(x)

[0274] or the group consisting of

[0275] a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x)

[0276] a chemo/electro-active material that comprisesNb_(a)Ti_(b)Zn_(c)O_(x), and

[0277] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x)

[0278] or the group consisting of

[0279] a chemo/electro-active material that comprisesNb_(a)Ti_(b)Zn_(c)O_(x)

[0280] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x),and

[0281] a chemo/electro-active material that comprises Ti_(a)Zn_(b)O_(x).

[0282] or the group consisting of

[0283] a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)

[0284] a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)

[0285] a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x)

[0286] a chemo/electro-active material that comprisesNb_(a)Ti_(b)Zn_(c)O_(x)

[0287] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x),and

[0288] a chemo/electro-active material that comprises Ti_(a)Zn_(b)O_(x).

[0289] or the group consisting of

[0290] a chemo/electro-active material that comprisesGa_(a)Ti_(b)Zn_(c)O_(x)

[0291] a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x),and

[0292] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)

[0293] or the group consisting of

[0294] a chemo/electro-active material that comprisesGa_(a)Ti_(b)Zn_(c)O_(x)

[0295] a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)

[0296] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)

[0297] a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x)

[0298] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x),and

[0299] a chemo/electro-active material that comprises Ti_(a)Zn_(b)O_(x).

[0300] or the group consisting of

[0301] a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x)

[0302] a chemo/electro-active material that comprises Fe_(a)Ti_(b)O_(x),and

[0303] a chemo/electro-active material that comprisesGa_(a)Ti_(b)Zn_(c)O_(x)

[0304] or the group consisting of

[0305] a chemo/electro-active material that comprises Fe_(a)Ti_(b)O_(x)

[0306] a chemo/electro-active material that comprisesGa_(a)Ti_(b)Zn_(c)O_(x), and

[0307] a chemo/electro-active material that comprises Nb_(a)W_(b)O_(x).

[0308] or the group consisting of

[0309] a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x)

[0310] a chemo/electro-active material that comprises Cu_(a)Ga_(b)O_(x),

[0311] a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x)

[0312] a chemo/electro-active material that comprises Fe_(a)Ti_(b)O_(x)

[0313] a chemo/electro-active material that comprisesGa_(a)Ti_(b)Zn_(c)O_(x), and

[0314] a chemo/electro-active material that comprises Nb_(a)W_(b)O_(x).

[0315] or the group consisting of

[0316] a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x)

[0317] a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x),and

[0318] a chemo/electro-active material that comprisesNb_(a)Ti_(b)Zn_(c)O_(x)

[0319] In the apparatus of this invention, a chemo/electro-activematerial that comprises M1Ox, a chemo/electro-active material thatcomprises M1aM2bOx, or a chemo/electro-active material that comprisesM1aM2bM3cOx may be selected from the group consisting of

[0320] a chemo/electro-active material that comprisesGa_(a)Ti_(b)Zn_(c)O_(x)

[0321] a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)

[0322] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x),and

[0323] a chemo/electro-active material that comprises SnO₂

[0324] or the group consisting of

[0325] a chemo/electro-active material that comprisesGa_(a)Ti_(b)Zn_(c)O_(x)

[0326] a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)

[0327] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)

[0328] a chemo/electro-active material that comprises SnO₂,

[0329] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x),and

[0330] a chemo/electro-active material that comprises Ti_(a)Zn_(b)O_(x).

[0331] or the group consisting of

[0332] a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x)

[0333] a chemo/electro-active material that comprisesNb_(a)Ti_(b)Zn_(c)O_(x), and

[0334] a chemo/electro-active material that comprises Pr₆O₁₁

[0335] or the group consisting of

[0336] a chemo/electro-active material that comprisesNb_(a)Ti_(b)Zn_(c)O_(x)

[0337] a chemo/electro-active material that comprises Pr₆O₁₁, and

[0338] a chemo/electro-active material that comprises Ti_(a)Zn_(b)O_(x)

[0339] or the group consisting of

[0340] a chemo/electro-active material that comprises Cr_(a)Mn_(b)O_(x)

[0341] a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x)

[0342] a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x)

[0343] a chemo/electro-active material that comprisesNb_(a)Ti_(b)Zn_(c)O_(x)

[0344] a chemo/electro-active material that comprises Pr₆O₁₁, and

[0345] a chemo/electro-active material that comprises Ti_(a)Zn_(b)O_(x).

[0346] In the apparatus of this invention, a chemo/electro-activematerial that comprises M1Ox, or a chemo/electro-active material thatcomprises M1aM2bOx may be selected from the group consisting of

[0347] a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)

[0348] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x),and

[0349] a chemo/electro-active material that comprises SnO₂.

[0350] or the group consisting of

[0351] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)

[0352] a chemo/electro-active material that comprises SnO₂, and

[0353] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x)

[0354] or the group consisting of

[0355] a chemo/electro-active material that comprises SnO₂,

[0356] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x),and

[0357] a chemo/electro-active material that comprises Ti_(a)Zn_(b)O_(x)

[0358] or the group consisting of

[0359] a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)

[0360] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)

[0361] a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x),and

[0362] a chemo/electro-active material that comprises ZnO.

[0363] or the group consisting of

[0364] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)

[0365] a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x)

[0366] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x),and

[0367] a chemo/electro-active material that comprises ZnO

[0368] or the group consisting of

[0369] a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x)

[0370] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x)

[0371] a chemo/electro-active material that comprises Ti_(a)Zn_(b)O_(x),and

[0372] a chemo/electro-active material that comprises ZnO

[0373] or the group consisting of

[0374] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x)

[0375] a chemo/electro-active material that comprises Ti_(a)Zn_(b)O_(x),and

[0376] a chemo/electro-active material that comprises ZnO.

[0377] or the group consisting of

[0378] a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)

[0379] a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)

[0380] a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x)

[0381] a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x)

[0382] a chemo/electro-active material that comprises Ti_(a)Zn_(b)O_(x),and

[0383] a chemo/electro-active material that comprises ZnO.

[0384] or the group consisting of

[0385] a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)

[0386] a chemo/electro-active material that comprises Cr_(a)Mn_(b)O_(x),and

[0387] a chemo/electro-active material that comprises CuO

[0388] or the group consisting of

[0389] a chemo/electro-active material that comprises Cr_(a)Mn_(b)O_(x)

[0390] a chemo/electro-active material that comprises CuO, and

[0391] a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x)

[0392] or group consisting of

[0393] a chemo/electro-active material that comprises CuO

[0394] a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x),and

[0395] a chemo/electro-active material that comprises Pr₆O₁₁

[0396] or group consisting of

[0397] a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x)

[0398] a chemo/electro-active material that comprises Pr₆O₁₁, and

[0399] a chemo/electro-active material that comprises WO₃.

[0400] or group consisting of

[0401] a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)

[0402] a chemo/electro-active material that comprises Cr_(a)Mn_(b)O_(x)

[0403] a chemo/electro-active material that comprises CuO

[0404] a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x)

[0405] a chemo/electro-active material that comprises Pr₆O₁₁, and

[0406] a chemo/electro-active material that comprises WO₃.

[0407] Any method of depositing the chemo/electro-active material to asubstrate is suitable. One technique used for deposition is applying asemiconducting material on an alumina substrate on which electrodes arescreen printed. The semiconducting material can be deposited on top ofelectrodes by hand painting semiconducting materials onto the substrate,pipetting materials into wells, thin film deposition, or thick filmprinting techniques. Most techniques are followed by a final firing tosinter the semiconducting materials.

[0408] Techniques for screen-printing substrates with the electrodes andchemo/electro-active materials are illustrated in FIGS. 2-3. FIG. 2depicts a method of using interdigitated electrodes overlaid withdielectric material, forming blank wells into which thechemo/electro-active materials can be deposited. FIG. 3 depicts anelectrode screen pattern for an array of 6 materials which is printed onboth sides of the substrate to provide for a 12-material array chip. Twoof the electrodes are in parallel so it holds only 6 unique materials.Counting down from the top of the array shown in FIG. 3, the top twomaterials can only be accessed simultaneously by the split electrodewith which they have shared contact. Below that is the screen patternfor the dielectric material, which is screen printed on top of theelectrodes on both sides of the substrate to prevent the material frombeing fouled by contact with the gas mixture, such as a deposit of sootthat could reduce the sensitivity of a sensor material to a gas or causea short. Below that is the screen pattern for the actual sensormaterials. This is printed in the holes in the dielectric on top of theelectrodes. When more than one material is used in the array, theindividual materials are printed one at a time.

[0409] The geometry of a sensor material as fabricated in an array,including such characteristics as its thickness, selection of a compoundor composition for use as the sensor, and the voltage applied across thearray, can vary depending on the sensitivity required. If desired, theapparatus may be constructed in a size such that it may be passedthrough an opening that is the size of a circle having a diameter of nomore than about 150 mm, or no more than about 100 mm, or no more thanabout 50 mm, or no more than about 25 mm, or no more than about 18 mm,as the requirements of it usage may dictate. The sensor materials arepreferably connected in parallel in a circuit to which a voltage ofabout 1 to about 20, preferably about 1 to about 12, volts is appliedacross the sensor materials.

[0410] As noted, the types of electrical response characteristics thatmay be measured include AC impedance or resistance, capacitance,voltage, current or DC resistance. It is preferred to use resistance asthe electric response characteristic of a sensor material that ismeasured to perform analysis of a gas mixture and/or a componenttherein. For example, a suitable sensor material may be that which, whenat a temperature of about 400° C. or above, has a resistivity of atleast about 1 ohm-cm, and preferably at least about 10 ohm-cm, and yetno more than about 10⁶ ohm-cm, preferably no more than about 10⁵ ohm-cm,and more preferably no more than about 10⁴ ohm-cm. Such a sensormaterial may also be characterized as that which exhibits, preferably ata temperature of about 400° C. or above, upon exposure to a gas mixture,a change in resistance of at least about 0.1 percent, and preferably atleast about 1 percent, as compared to the resistance in the absence ofexposure. Using such material, a signal may be generated that isproportional to the resistance of exhibited by the material when it isexposed to a multi-component gas mixture.

[0411] Regardless of the type of response characteristic that ismeasured for the purpose of analyzing a mixture and/or a gaseouscomponent of interest therein, it is desirable that a sensor material beutilized for which a quantified value of that response characteristic isstable over an extended period of time. When the sensor material isexposed to a mixture containing the analyte, the concentration of theanalyte being a function of the composition of the particular gasmixture in which it is contained, the value of the response of thesensor material will preferably remain constant or vary to only a smallextent during exposure to the mixture over an extended period of time ata constant temperature. For example, the value of the response, if itvaries, will vary by no more than about twenty percent, preferably nomore than about ten percent, more preferably no more than about fivepercent, and most preferably no more than about one percent over aperiod of at least about 1 minute, or preferably a period of hours suchas at least about 1 hour, preferably at least about 10 hours, morepreferably at least about 100 hours, and most preferably at least about1000 hours. One of the advantages of the types of sensor materialsdescribed above is that they are characterized by this kind of stabilityof response.

[0412] The electrical response characteristic exhibited by achemo/electro-active material in respect of a multi-component gasmixture that contains an analyte gas or sub-group of gases derives fromcontact of the surface of the chemo/electro-active material with the gasmixture containing the analyte(s). The electrical responsecharacteristic is an electrical property, such as capacitance, voltage,current, AC impedance, or AC or DC resistance, that is affected byexposure of the chemo/electro-active material to the multi-component gasmixture. A quantified value of, or a signal proportional to thequantified value of, the electrical property or a change in theelectrical property may be obtained as a useful measurement at one ormore times while the material is exposed to the gas mixture.

[0413] An electrical response is determined for eachchemo/electro-active material upon exposure of the array to a gasmixture, and means for determining the response include conductorsinterconnecting the sensor materials. The conductors are in turnconnected to electrical input and output circuitry, including dataacquisition and manipulation devices as appropriate to measure andrecord a response exhibited by a sensor material in the form of anelectrical signal. The value of a response, such as a measurementrelated to resistance, may be indicated by the size of the signal. Oneor more signals may be generated by an array of sensors as to eachanalyte component in the mixture, whether the analyte is one or moreindividual gases and/or one or more subgroups of gases.

[0414] An electrical response is determined for each individualchemo/electro-active material separately from that of each of the otherchemo/electro-active materials. This can be accomplished by accessingeach chemo/electro-active material with an electric currentsequentially, using a multiplexer to provide signals differentiatedbetween one material and another in, for example, the time domain orfrequency domain. It is consequently preferred that nochemo/electro-active material be joined in a series circuit with anyother such material. One electrode, by which a current is passed to achemo/electro-active material, can nevertheless be laid out to havecontact with more than one material. An electrode may have contact withall, or fewer than all, of the chemo/electro-active materials in anarray. For example, if an array has 12 chemo/electro-active materials,an electrode may have contact with each member of a group of 2, 3, 4, 5or 6 (or, optionally, more in each instance) of the chemo/electro-activematerials. The electrode will preferably be laid out to permit anelectrical current to be passed to each member of such group ofchemo/electro-active materials sequentially.

[0415] A conductor such as a printed circuit may be used to connect avoltage source to a sensor material, and, when a voltage is appliedacross the sensor material, a corresponding current is created throughthe material. Although the voltage may be AC or DC, the magnitude of thevoltage will typically be held constant. The resulting current isproportional to both the applied voltage and the resistance of thesensor material. A response of the material in the form of either thecurrent, voltage or resistance may be determined, and means for doing soinclude commercial analog circuit components such as precisionresistors, filtering capacitors and operational amplifiers (such as aOPA4340). As voltage, current and resistance is each a known function ofthe other two electrical properties, a known quantity for one propertymay be readily converted to that of another.

[0416] Resistance may be determined, for example, in connection with thedigitization of an electrical response. Means for digitizing anelectrical response include an analog to digital (A/D) converter, asknown in the art, and may include, for example, electrical componentsand circuitry that involve the operation of a comparator. An electricalresponse in the form of a voltage signal, derived as described above asa result of applying a voltage across a sensor material, is used as aninput to a comparator section (such as a LM339). The other input to thecomparator is driven by a linear ramp produced by charging a capacitorusing a constant current source configured from an operational amplifier(such as a LT1014) and an external transistor (such as a PN2007a). Theramp is controlled and monitored by a microcomputer (such as aT89C51CC01). A second comparator section is also driven by the rampvoltage, but is compared to a precise reference voltage. Themicrocomputer captures the length of time from the start of the ramp tothe activation of the comparators to generate a signal based on thecounted time.

[0417] The resistance of the sensor material is then calculated, orquantified as a value, by the microcomputer from the ratio of the timesignal derived from the voltage output of the material to a time signalcorresponding to a known look-up voltage and, ultimately, to theresistance that is a function of the look-up voltage. A microprocessorchip, such as a T89C51CC01, can be used for this function. Themicroprocessor chip may also serve as means for determining a change inthe resistance of a sensor material by comparing a resistance,determined as above, to a previously determined value of the resistance.

[0418] Electrical properties such as impedance or capacitance may bedetermined, for example, by the use of circuitry components such as animpedance meter, a capacitance meter or inductance meter.

[0419] Means for digitizing the temperature of an array ofchemo/electro-active materials can include, for example, components asdescribed above that convert a signal representative of a physicalproperty, state or condition of a temperature-measuring device to asignal based on counted time.

[0420] In one embodiment, analysis of a multi-component gas mixture iscomplete upon the generation of an electrical response, such asresistance, in the manner described above. As a measurement ofresistance exhibited by a sensor material upon exposure to a gas mixtureis a function of the partial pressure within the mixture of one or morecomponent gases, the measured resistance provides useful informationabout the composition of the gas mixture. The information may, forexample, indicate the presence or absence within the mixture of aparticular gas or subgroup of gases. In other embodiments, however, itmay be preferred to manipulate, or further manipulate, an electricalresponse in the manner necessary to obtain information related to theconcentration within the mixture of one or more particular componentgases or subgroups of gases, or to calculate the actual concentrationwithin the mixture of one or more component gases or subgroups.

[0421] Means for obtaining information concerning the relativeconcentration within the mixture of one or more individual componentgases and/or one or more subgroups of gases, or for detecting thepresence of, or calculating the actual concentration of, one or moreindividual component gases and/or subgroups within the mixture, mayinclude a modeling algorithm that incorporates either a PLS (Projectiononto Latent Systems) model, a back-propagation neural network model, ora combination of the two, along with signal pre-processing and outputpost-processing. Signal pre-processing includes, but is not limited to,such operations as principle component analyses, simple lineartransformations and scaling, logarithmic and natural logarithmictransformations, differences of raw signal values (e.g., resistances),and differences of logarithmic values. The algorithm contains a modelwhose parameters have been previously determined, and that empiricallymodels the relationship between the pre-processed input signal andinformation related to the gas concentration of the species of interest.Output post-processing includes, but is not limited to, all of theoperations listed above, as well as their inverse operations.

[0422] The model is constructed using equations in which constants,coefficients or other factors are derived from pre-determined valuescharacteristic of a precisely measured electrical response of anindividual sensor material to a particular individual gas or subgroupexpected to be present as a component in the mixture to be analyzed. Theequations may be constructed in any manner that takes temperature intoaccount as a value separate and apart from the electrical responsesexhibited by the sensor materials upon exposure to a gas mixture. Eachindividual sensor material in the array differs from each of the othersensors in its response to at least one of the component gases orsubgroups in the mixture, and these different responses of each of thesensors is determined and used to construct the equations used in themodel.

[0423] A change of temperature in the array may be indicated by a changein the quantified value of an electrical response characteristic,resistance for example, of a sensor material. At a constant partialpressure in the mixture of a gas of interest, the value of an electricalresponse characteristic of a sensor material may vary with a change intemperature of the array, and thus the material. This change in thevalue of an electrical response characteristic may be measured for thepurpose of determining or measuring the extent of change of, and thus avalue for, temperature. The temperature of the array will be the same,or substantially the same, as the temperature of the gas mixture unlessthe array is being maintained at a pre-selected temperature by a heaterlocated on the substrate. If the array is being heated by a heater, thetemperature of the array will lie substantially in the range withinwhich the heater cycles on and off.

[0424] It is not required, but is preferred, that the measurement oftemperature be made independently of information related to thecompositional content of a gas mixture. This can be done by not usingsensors that provide compositional information for the additionalpurpose of determining temperature, and, optionally, by connecting thetemperature measuring device in parallel circuitry with the sensormaterials, rather than in series. Means for measuring temperatureinclude a thermocouple or a pyrometer incorporated with an array ofsensors. If the termperature determining device is a thermistor, whichis typically a material that is not responsive to an analyte gas, thethermistor is preferably made from a different material than thematerial from which any of the gas sensors is made. Regardless of themethod by which temperature or change in temperature is determined, atemperature value or a quantified change in temperature is a desirableinput, preferably in digitized form, from which an analysis of a mixtureof gases and/or a component therein may be performed.

[0425] In the method and apparatus of this invention, unlike variousprior-art technologies, there is no need to separate the component gasesof a mixture for purposes of performing an analysis, such as by amembrane or electrolytic cell. There is also no need when performing ananalysis by means of this invention to employ a reference gas externalto the system, such as for the purpose of bringing a response oranalytical results back to a base line value. A value representative ofa reference state may, however, be used as a factor in an algorithm bywhich information related to the composition of the gas mixture isdetermined. With the exception of preliminary testing, during which astandardized response value to be assigned to the exposure of eachindividual sensor material to each individual analyte gas is determined,the sensor materials are exposed only to the mixture in which an analytegas and/or subgroup is contained. The sensor materials are not exposedto any other gas to obtain response values for comparison to thoseobtained from exposure to the mixture containing an analyte. Theanalysis of the mixture is therefore performed only from the electricalresponses obtained upon exposure of the chemo/electro-active materialsto the mixture containing the analyte. No information about an analytegas and/or subgroup is inferred by exposure of the sensor materials toany gas other than the analyte itself as contained within the mixture.

[0426] This invention is therefore useful at the higher temperaturesfound in automotive emission systems, typically in the range of fromabout 400° C. to about 1000° C. In addition to gasoline and dieselinternal combustion engines, however, there is a variety of othercombustion processes to which this invention could be applied, includingstack or burner emissions of all kinds such as resulting from chemicalmanufacturing, electrical generation, waste incineration and airheating. These applications require the detection of gases such asnitrogen oxides, ammonia, carbon monoxide, hydrocarbons and oxygen atthe ppm to percent levels, typically in a highly corrosive environment.

[0427] When the multi-component gas mixture comprises a nitrogen oxide,a hydrocarbon, or both, or any of the other gases mentioned herein, theapparatus may be used to determine the presence and/or concentration ofa nitrogen oxide and/or hydrocarbon in the multi-component gas mixture.The apparatus may also be used to determine the presence and/orconcentration of any one or more to the other gases mentioned hereinthat may be present in a multi-component gas mixture. For this purpose,the electrical response, in the apparatus of this invention, of one ormore of a chemo/electro-active material that comprises M¹O_(x), achemo/electro-active material that comprises M¹ _(a)M² _(b)O_(x), and achemo/electro-active material that comprises M¹ _(a)M² _(b)M³ _(c)O_(x)may be related to one or more of the presence of a nitrogen oxide withinthe gas mixture, the presence of a hydrocarbon within the gas mixture,the collective concentration of all nitrogen oxides within the gasmixture, and the concentration of a hydrocarbon within the gas mixture.

[0428] This invention therefore provides methods and apparatus fordirectly sensing the presence and/or concentration of one or more gasesin an multi-component gas system, comprising an array of at least twochemo/electro-active materials chosen to detect analyte gases orsubgroups of gases in a multi-component gas stream. The multi-componentgas system can be at essentially any temperature that is not so low orso high that the sensor materials are degraded or the sensor apparatusotherwise malfunctions. In one embodiment, the gas system may be at alower temperature such as room temperature (about 25° C.) or elsewherein the range of about 0° C. to less than about 100° C., whereas in otherembodiments the gas mixture may at a higher temperature such as in therange of about 400° C. to about 1000° C. or more. The gas mixture maytherefore have a temperature that is about 0° C. or more, about 100° C.or more, about 200° C. or more, about 300° C. or more, about 400° C. ormore, about 500° C. or more, about 600° C. or more, about 700° C. ormore, or about 800° C. or more, and yet is less than about 1000° C., isless than about 900° C., is less than about 800° C., is less than about700° C., is less than about 600° C., is less than about 500° C., is lessthan about 400° C., is less than about 300° C., is less than about 200°C., or is less than about 100° C.

[0429] In applications in which the gas mixture is above about 400° C.,the temperature of the sensor materials and the array may be determinedsubstantially only, and preferably is determined solely, by thetemperature of the gas mixture in which a gaseous analyst is contained.This is typically a variable temperature. When higher-temperature gasesare being analyzed, it may be desirable to provide a heater with thearray to bring the sensor materials quickly to a minimum temperature.Once the analysis has begun, however, the heater (if used) is typicallyswitched off, and no method is provided to maintain the sensor materialsat a preselected temperature. The temperature of the sensor materialsthus rises or falls to the same extent that the temperature of thesurrounding environment does. The temperature of the surroundingenvironment, and thus the sensors and the array, is typically determinedby (or results from) substantially only the temperature of the gasmixture to which the array is exposed.

[0430] In applications in which the gas mixture is below about 400° C.,it may be preferred to maintain the sensor materials and the array at apreselected temperature of about 200° C. or above, and preferably 400°C. or above. This preselected temperature may be substantially constant,or preferably is constant. The preselected temperature may also be about500° C. or above, about 600° C. or above, about 700° C. or above, about800° C. or above, about 900° C. or above, or about 10001C or above. Thismay be conveniently done with a heater incorporated with the array, in amanner as known in the art. If desired, a separate micro heater meansmay be supplied for each separate chemo/electro-active material, and anyone or more of the materials may be heated to the same or a differenttemperature. The temperature of the gas mixture in such case may also bebelow about 300° C., below about 200° C., below about 100° C., or belowabout 50° C. In these low temperature application, the means for heatingthe chemo/electro-active materials may be a voltage source that has avoltage in the range of about 10⁻³ to about 10⁻⁶ volts. The substrate onwhich the materials are placed may be made of a materials that isselected from one or more of the group consisting of silicon, siliconcarbide, silicon nitride, and alumina containing a resistive dopant.Devices used in these low temperature applications are often smallenough to be held in the human hand.

[0431] This heating technique is also applicable, however, to theanalysis of high temperature gases. When the temperature of the gasmixture is above about 400° C., the sensor materials may nevertheless bemaintained by a heater at a constant or substantially constantpreselected temperature that is higher than the temperature of the gasmixture. Such preselected temperature may be about 500° C. or above,about 600° C. or above, about 700° C. or above, about 800° C. or above,about 900° C. or above, or about 1000° C. or above. Should thetemperature of the gas mixture exceed the temperature pre-selected forthe heater, the heater may be switched off during such time. Atemperature sensor will still be employed, however, to measure thetemperature of the gas mixture and provide that value as an input to analgorithm by which information related to the composition of the gasmixture is determined.

[0432] In summary, it may be seen that this invention provides means todetermine, measure and record responses exhibited by each of thechemo/electro-active materials present in an array upon exposure to agas mixture. Any means that will determine, measure and record changesin electrical properties can be used, such as a device that is capableof measuring the change in AC impedance of the materials in response tothe concentration of adsorbed gas molecules at their surfaces. Othermeans for determining electrical properties are suitable devices tomeasure, for example, capacitance, voltage, current or DC resistance.Alternatively a change in temperature of the sensing material may bemeasured and recorded. The chemical sensing method and apparatus mayfurther provide means to measure or analyze a mixture and/or thedetected gases such that the presence of the gases are identified and/ortheir concentrations are measured. These means can includeinstrumentation or equipment that is capable, for example, of performingchemometrics, neural networks or other pattern recognition techniques.The chemical sensor apparatus will further comprise a housing for thearray of chemo/electro-active materials, the means for detecting, andmeans for analyzing.

[0433] The device includes a substrate, an array of at least twochemo/electro-active materials chosen to detect one or morepredetermined gases in a multi-component gas stream, and a means todetect changes in electrical properties in each of thechemo/electro-active materials present upon exposure to the gas system.The array of sensor materials should be able to detect an analyte ofinterest despite competing reactions caused by the presence of theseveral other components of a multi-component mixture. For this purpose,this invention uses an array or multiplicity of sensor materials, asdescribed herein, each of which has a different sensitivity for at leastone of the gas components of the mixture to be detected. A sensor thathas the needed sensitivity, and that can operate to generate the typesof analytical measurements and results described above, is obtained byselection of appropriate compositions of materials from which the sensoris made. Various suitable types of materials for this purpose aredescribed above. The number of sensors in the array is typically greaterthan or equal to the number of individual gas components to be analyzedin the mixture.

[0434] Further description relevant to the apparatus of this invention,uses for the apparatus and methods of using the apparatus may be foundin U.S. Provisional Application No. 60/370,445, filed Apr. 5, 2002, andU.S. application Ser. No. 10/117,472, filed Apr. 5, 2002, each of whichis incorporated in its entirety as a part hereof for all purposes.

What is claimed is:
 1. An apparatus for reducing a nitrogen oxidecontained in a multi-component gas mixture emitted by a emissionssource, comprising (a) an exhaust conduit for transporting the gasmixture downstream from the emissions source, (b) an injector forinjecting a reducing agent into the conduit, and (c) one or more gasanalyzers located in the conduit downstream of the injector.
 2. Anapparatus according to claim 1 further comprising a catalyst to catalyzethe reduction of the nitrogen oxide.
 3. An apparatus according to claim2 wherein a catalyst is located upstream from a gas analyzer.
 4. Anapparatus according to claim 2 wherein a catalyst is located downstreamfrom a gas analyzer.
 5. An apparatus according to claim 2 wherein afirst catalyst is located upstream from a gas analyzer, and a secondcatalyst is located downstream from the gas analyzer.
 6. An apparatusaccording to claim 5 wherein a catalyst comprises a plurality ofvertically arranged catalyst beds, and a first catalyst bed is locatedvertically upstream from a gas analyzer, and a second catalyst bed islocated vertically downstream from the gas analyzer.
 7. An apparatusaccording to claim 2 wherein a first gas analyzer is located upstreamfrom a catalyst, and a second gas analyzer is located downstream fromthe catalyst.
 8. An apparatus according to claim 1 which comprises aplurality of gas analyzers.
 9. An apparatus according to claim 2 whichcomprises a plurality of gas analyzers, and wherein a first catalyst islocated upstream from a plurality of gas analyzers, and a secondcatalyst is located downstream from the plurality of gas analyzers. 10.An apparatus according to claim 1 further comprising one or more gasanalyzers located in the conduit upstream from the injector.
 11. Anapparatus according to claim 10 further comprising a catalyst tocatalyze the reduction of the nitrogen oxide.
 12. An apparatus accordingto claim 11 wherein a catalyst is located upstream from a gas analyzer.13. An apparatus according to claim 11 wherein a catalyst is locateddownstream from a gas analyzer.
 14. An apparatus according to claim 11wherein a first catalyst is located upstream from a gas analyzer, and asecond catalyst is located downstream from the gas analyzer.
 15. Anapparatus according to claim 14 wherein a catalyst comprises a pluralityof vertically arranged catalyst beds, and a first catalyst bed islocated upstream from a gas analyzer, and a second catalyst bed islocated downstream from the gas analyzer.
 16. An apparatus according toclaim 11 wherein a first gas analyzer is located upstream from acatalyst, and a second gas analyzer is located downstream from thecatalyst.
 17. An apparatus according to claim 11 that comprises aplurality of gas analyzers, and wherein a first catalyst is locatedupstream from a plurality of gas analyzers, and a second catalyst islocated downstream from the plurality of gas analyzers.
 18. An apparatusaccording to claim 1, 2, 10 or 11 wherein a gas analyzer comprises anarray of chemo/electro-active materials.
 19. An apparatus according toclaim 1, 2, 10 or 11 wherein a gas analyzer outputs at least one signalthat is related to the individual concentration within the gas mixtureof an individual gas component therein.
 20. An apparatus according toclaim 1, 2, 10 or 11 wherein a gas analyzer outputs at least one signalthat is related to the collective concentration within the gas mixtureof a subgroup of the component gases therein.
 21. An apparatus accordingto claim 1, 2, 10 or 11 wherein a gas analyzer outputs at least onesignal that is related to the individual concentration within the gasmixture of an individual gas component therein, and at least one signalthat is related to the collective concentration within the gas mixtureof a subgroup of the component gases therein.
 22. An apparatus accordingto claim 1, 2, 10 or 11 wherein a gas analyzer outputs a signal to adecision-making routine.
 23. An apparatus according to claim 1, 2, 10 or11 wherein a gas analyzer that is upstream from all catalyst, and a gasanalyzer that is downstream from all catalyst, both output a signal to adecision-making routine.
 24. An apparatus according to claim 1, 2, 10 or11 wherein a gas analyzer that is upstream from all catalyst, a gasanalyzer that is downstream from a first catalyst and upstream from asecond catalyst, and a gas analyzer that is downstream from allcatalyst, each outputs a signal to a decision-making routine.
 25. Anapparatus according to claim 1, 2, 10 or 11 wherein the gas analyzeroutputs a signal to a map.
 26. An apparatus according to claim 1, 2, 10or 11 wherein a gas analyzer outputs a signal to a decision-makingroutine that controls the injection of reducing agent.
 27. An apparatusaccording to claim 1, 2, 10 or 11 wherein a gas analyzer outputs asignal to a decision-making routine that calculates an amount ofreducing agent to be injected.
 28. An apparatus according to claim 1, 2,10 or 11 wherein a gas analyzer outputs at least one signal that isrelated to the individual concentration within the gas mixture of anindividual nitrogen oxide component therein.
 29. An apparatus accordingto claim 1, 2, 10 or 11 wherein a gas analyzer outputs at least onesignal that is related to the collective concentration within the gasmixture of all nitrogen oxide components therein.
 30. An apparatusaccording to claim 1, 2, 10 or 11 wherein a gas analyzer outputs atleast one signal that is related to the individual concentration withinthe gas mixture of one or more or all nitrogen oxide components therein,and the signal is outputted to a decision-making routine that calculatesan amount of reducing agent to be injected.
 31. An apparatus accordingto claim 1, 2, 10 or 11 wherein the reducing agent is ammonia.
 32. Anapparatus according to claim 1, 2, 10 or 11 wherein the reducing agentis urea.
 33. An apparatus according to claim 1, 2, 10 or 11 wherein thecombustion source is stationary.
 34. An electrical generating plantcomprising an apparatus for reducing a nitrogen oxide gas according toclaim 1, 2, 10 or
 11. 35. A furnace comprising an apparatus for reducinga nitrogen oxide gas according to claim 1, 2, 10 or
 11. 36. A steamturbine comprising an apparatus for reducing a nitrogen oxide gasaccording to claim 1, 2, 10 or
 11. 37. A gas turbine comprising anapparatus for reducing a nitrogen oxide gas according to claim 1, 2, 10or 11.